London and Amsterdam--should I add small town in England or Germany?

Time of year would be most helpful, as well as knowing the types of things you are interested in seeing or doing. Without knowing that, I'm not sure why you would want to add extra travel time by going out of your way for just a couple of days in Germany. England offers many choices, from Bath or Oxford, all the way to York (one of my favorite places). If you are going to Bruges, you could easily spend a couple of days in Ghent. Or in the Netherlands, how about Delft.

Late June/early July. We would be interested in seeing castles, ruins, quaint villages, half-timbered towns. Thanks!

Aachen is not a small town but a beloved tourist destination not too far away from Bruges.

Xanten is not too far from Amsterdam and has Roman roots.

For Germany, I would recommend Bacharach on the Rhine River. Other popular towns with tourists on the Rhine are St. Goar, St. Goarshausen, Boppard, Koblenz and Kaub.

As for Bacharach, the town gets its name from Bacchus, the Roman god of wine. A Medieval Pope ordered his wine from Bacharach. The surrounding area produces superb wines. It's ironic because Italy and France are synonymous with wine, but the best wine I had in Europe was in Bacharach. Weingut Fritz Bastian, an eatery in Bacharach, features what it calls the wine wheel. They put five to seven different wine types in a wheel at the table and teach guests about each one. I rented a bicycle from a storefront on the Rhine and bicycled along the river for at least 50 miles. Finally, Bacharach has an authentic Medieval castle, Burg Stahleck, https://www.romantischer-rhein.de/en/a-burg-stahleck, that houses a youth hostel. In 1996, when I was in my early 30s, I stayed there. I woke up, looked out the window of my dorm room, and there was the Rhine River.

Finally, the Rhine is served my multiple boat lines that stop in Bacharach. You can go up or down the river for a price of a ticket. A popular place to go is the Cologne to see the breathtaking Cologne Cathedral -- still one of the best cathedrals of the 50 or so I've seen in Europe -- and the Roman museum, https://roemisch-germanisches-museum.de/Homepage. The cathedral hosts frequent organ concerts.

If it were me, I would look at nice places to go not too far from London. I think you could have a really nice time in Kent- Canterbury has half-timbered buildings (plus the amazing cathedral) and that time of year is a great time to visit the Kent coast- places like Whitstable or Broadstairs. There's a big castle nearby as well (Leeds Castle) which while relatively modern was built on the site of a medieval castle. You can also get the train from Canterbury directly back to St Pancras in London if you were planning to get the Eurostar next to depart for Amsterdam.

Since you are already in London it would be more convenient to visit the English countryside.
I recommend several choices:
1. Bath would be excellent
2. Stratford Upon Avon if you are a fan of William Shakespeare.
3. Oxford or Cambridge for famous universities.
4.Canterbury

Also, consider a day going to Windsor Castle and Stonehenge.

lorinela,
London alone can fill up a week easily, with another week in the countryside of England. (Wales, by the way, has more castles per person than any place else, or so I have been told.) Two days in Wales can give you the small town you desire and castles galore, plus the suggestions of geovagriffith for other places in England can fill the second week.

However, if you are heading to Amsterdam, I would take the Eurostar to Amsterdam and spend four full days there. Give ten days to England. You will need it.
Be sure to figure nights in a place, not days, as travel between locations can use up several hours of a day. Two nights is one full day of sightseeing. Have you discounted your arrival day as not a day for much touring? Likewise departure day. After these two days, how many days do you have left? If it is 12, plan your time accordingly, and subtract a day for your travel from London to Amsterdam, thus 11 days for actual sightseeing.
Be sure to book your flights open jaw (not two one ways) and fly home from Amsterdam to save backtracking.

Have fun in planning!

"...castles, ruins, quaint villages, half-timbered towns."

Cologne and Aachen are fine places to visit in Germany but not so great for what you have in mind. I think you should set your sights on the Upper Middle Rhine Valley, that specific part of the Rhine River which Craig has posted about. It is this river segment - Koblenz to Bingen - which contains the things you are looking for and which has been designated a UNESCO World Heritage site for its historical and cultural significance.

"We'd rather spend a few nights at one location vs. day trips to multiple locations."

Well, a few nights in one location will work out fine. But to see the area, you simply MUST make day trips (short ones) to other towns and sights along the river - which is a very easy thing to do. Trains serve both river banks, ferry crossings get you to the other side, and cruise boats can provide views of the dozens of castles and castle ruins that dot the cliffsides. See maps below.

Transport map
Castles map

CASTLES, HALF-TIMBERED BUILDINGS:
The castle Craig mentions in Bacharach (Stahleck) can be seen in this photo. It's impressive from the river for sure, but as Craig explains, its interior has been chopped up into modernized bedrooms that house hostelers... nothing to see inside, really... so for a castle tour, I will suggest instead MARKSBURG Castle in the town of BRAUBACH. This one is the single most solidly medieval castle in the entire Rhine Valley; tours are daily with English tours at 1 and 4 pm. Did you click on the link to Braubach? There are 3 pages with photos of this totally stunning half-timbered town. Be sure to walk the town when you go there.

Map of Braubach ("Bahnhof" near the map's center is the train station."

You should of course visit Bacharach as well - it also has stunning half-timbered buildings - but the walk up to Stahleck Castle is exhausting and not worth the trouble.

Auf Schönburg Castle in Oberwesel: A major top-end splurge castle-hotel. If you stay there, plan on a hefty hike using a taxi to get down to the railway.

RUINS:
Rheinfels Castle in St Goar is an amazing set of ruins with a small museum. Do see it.
Oberwesel's town walll is partially ruined - there's a walking trail that follows the wall all around town:
https://www.niederrheinscout.com/wp-content/uploads/2020/08/Stadtmauerrundgang-Oberwesel.jpg
Roman Fort in Boppard: Not huge, but easy to see and near the town center.

OTHER TOWNS to visit:
Rüdesheim (pretty touristy but fun)
Osterspai (very quiet town, attractive half-timbered buildings)
Boppard (awesome chairlift ride with terrace-restaurant (Gedeonseck) up top and fabulous river views

Where to stay? My pick is Boppard. Many hotel and dining choices, some on the river. And all guests get a free pass for local train trips!

I agree with CJean. London deserves a full week and you could easily do a day trip (or spend a couple of days) out of there to places like the Cotswolds, which has quaint towns galore. And it doesn't have to be the Cotswolds. Here is one article that have some suggestions for eight beautiful and quaint villages that are a quick train trip from London. https://www.discoverbritain.com/journey-planning/travel-guide/top-8-day-trips-out-of-london/

Then head to Amsterdam spending three nights there and the remainder of your time in Bruges. I agree about adding in a day or two in Ghent which is wonderful.

While I love the Rhine area, I don't think it's really feasible for you on this trip. You'd be better off sticking with the three areas you've mentioned and seeing what's around there. You can find quaint towns galore in England.

If lorinela indeed has "a few nights" to play with as stated, I don't understand why the Rhine Valley isn't feasible.

Using Koblenz (gateway to the the Rhine) as an in/out location, it is 4 to 4.5 train hours from either Bruges or A'dam. After the Rhine, fly to London (or home) out of Cologne/Bonn or FRA airport... both airports are 1.5 hours from Koblenz.

I could easily see 5 nights in London, 3-4 in A'dam, 2 in Bruges, 3 on the Rhine.

Cardiff is an easy train ride from London and you could go to the St. Fagan's outdoor Museum of Welsh Life.

Thank you everyone who weighed in—your input is greatly appreciated! We have decided on the Rhine Valley for a few days. Another question I have is if we have a rail pass, is it reasonable to think we could stop in Belgium (Bruges or Brussels or Ghent) on our way from London to Amsterdam for the afternoon? Signed, inexperienced traveler.

are you using your railpass - which one, Eurailpass or something different - global or regional - for the Brussels > Amsterdam leg?

Are your reservations far enough apart for that sidetrip?

Can you give a weensie more detail please?

Thank you Nigel. Yes we plan to purchase Eurail Global passes. We would leave London in the morning and have accommodations in Amsterdam that same night. So we're wondering if we could hop off the train for a few hours in say Bruges or Brussels and hop back on and continue onto Amsterdam. Or perhaps that is too much and we should either spend a night or two in Belgium or else skip Belgium altogether. Thank you for your input.

I have not been to Bruges, but I would not waste a night of your time on Brussels.

even with that pass you need to pay for reservations - not cheap - on both legs if you stick with Eurostar. You must use them to travel through the Channel Tunnel, and they are the fastest Brussels to Amsterdam. There is a Dutch/Belgian new train which makes an alternative between Brussels and Amsterdam but it makes a few more stops.

It makes your travel day quite long if you take the hour plus time on the ground plus an hour back to Brussels from Bruges.

I must question the wisdom of a railpass for your plans. You can find saver fares right now at DB for Amst > Kobl for €20-60 each for June dates. London > Brussels should be around €50 (one way) or 80 (round trip) each w/ advance purchase.

https://www.seat61.com/trains-and-routes/london-to-paris-by-eurostar.htm#fares

What about an Amst > Kobl stopover?... a stop in Brussels might work - check the schedules and fares. Here's a suggestion that might be right for you...

https://www.inzutphen.nl/en

We took advantage of some RS forum advice 2 years ago by making a stopover between Amsterdam and Koblenz in the lovely Dutch town of Zutphen, another Hanseatic-League town that reminded us a LOT of Bruges - just without the mob of international tourists that tends to overtake Bruges. The travel times are good. And one-way DB saver fares for late June for Amst > Kobl w/ the stopover - whether brief or overnight - are in the same €20-60 range as without the stopover. Approximate travel times:

1.3 hrs. Amst > Zutphen (9:31 - 10:54)
3.7 hrs. Zutphen > Koblenz

With a railpass, an overnight stopover anywhere will eat up 2 of your allotted travel days.

And if you stay in Boppard, your local train travel will be mostly free, as indicated.

https://www.vrminfo.de/en/tickets/tickets/leisure-tickets/vrm-guest-ticket/

Train travel is the other piece that I am trying to research and figure out. Thank you for the link to Seat 61. You all are so wonderful!