Flying to Frankfort & travel toHeidelberg & Manheim

Google Maps' reference centres for Heidelberg and Mannheim are only 12 miles apart, main station to main station is a 16 minute ride on a local train, hardly seems worth the hassle of moving hotels. Mannheim has regular direct trains from Frankfurt airport which would make it simpler but Heidelberg is more of a tourist destination.

I would not bother moving, the cities are so close together. Mannheim is not very touristy, and not that much to see there. I would stay the whole time in Heidelberg.
Come into Frankfurt on the S-bahn and catch one of the trains to Heidelberg that run all the time.

With 10 days altogether I assume you'll be visiting other places besides these two cities.

Heidelberg is a reasonably good base town, depending on where you stay and what outings you intend to take. It's more atmospheric and inviting than Mannheim. But Mannheim is a major train hub to other places.

Before advising you further, I'd have to know what outings you have in mind.

We will be staying in Frankfort 1 night after arrival & 1 night before departure so will stay 2 nights there total.
Also my wife has 1 brother who was born in Mannheim & her mother was born & lived there for many years so want to spend some time there.
We would like to do a couple of different tours along the Rhine River also & sounds like most of those tours are out of Heidelberg.

Heidelberg is on the Neckar, not the Rhein. If you want to go on the Rhein and see the castles, etc, do that from Frankfurt, not from Heidelberg.

OK, that information helps a little.

"Rhine Tour" - There may in fact be some kind of tour that would take you from Heidelberg or Mannheim all the way to the Middle Rhine Valley (the scenic part of the Rhine Valley and back again, but I'm not aware of it. Most daytrippers to Rhine do as follows:

• Train to Bingen or Rüdesheim to board a Rhine cruise ship
• Cruise north to St Goar or St Goarshausen
• Take a walk through one or two old-world towns

A base in Mannheim would work better than Heidelberg for such an outing. Sample schedule:

• Train from Mannheim to Bingen Rhein Stadt station: 10:15 - 11:56; walk 5 minutes to the waterfront, buy cruise tickets, enjoy the views, have lunch at one of the restaurants there, take a stroll along the promenade there.

• https://riverside-bingen.de/wp-content/uploads/2022/03/MLB.jpg

• Cruise to St Goar, 14:00 - 15:45 with B-R cruises; catch train to Bacharach (15:56 - 16:05) for a walk around this handsome town; find dinner.

https://www.mittelrheinentdecken.de/en/villages-towns-and-cities/bacharach/
https://www.informagiovani-italia.com/mappa_bacharach_am_rhein2.jpg

• Train back to Mannheim:

18:06 - 20:07
19:06 - 21:14
19:36 - 21:42

Fares:
€26 per person for B-R cruise
€38 day pass for two adults covers all train trips

I imagine you might need some hours in Mannheim for hunting down the former addresses of your family, etc. Mannheim also has a few things that might interest you:

Palace: you can walk there from the station...

https://www.tripadvisor.com/Attraction_Review-g187290-d2323352-Reviews-Baroque_Palace_Mannheim-Mannheim_Baden_Wurttemberg.html

https://www.schloesser-und-gaerten.de/fileadmin/Broschueren/monumentflyer_2023-24_englisch/ssg_barockschloss-mannheim_monumentbroschuere-GB_2023-24.pdf

Other:

https://www.tripadvisor.com/Attractions-g187290-Activities-oa0-Mannheim_Baden_Wurttemberg.html

NEAR MANNHEIM: The cities of Speyer and Worms are good places to visit and only about 25 minutes away by direct train. If you have time for only one, Speyer has the most to see/do, I think.

https://www.tripadvisor.com/Attractions-g198634-Activities-oa0-Speyer_Rhineland_Palatinate.html
https://www.tripadvisor.com/Attractions-g198632-Activities-oa0-Worms_Rhineland_Palatinate.html

So I can see the value of staying in Mannheim for a few nights.

With the other nights you will have in Heidelberg, some short outings are possible as well.

The Bergstrasse old-world wine towns are very close by and accessible by train; Ladenburg is super-close to Heidelberg, Heppenheim extra nice:

https://diebergstrasse.de/storage/2025/04/Holiday-Route-2024.pdf

The town of Bad Wimpen (which has been honored as Germany's most beautiful old town) is just one of several nice towns that share the Neckar River with Heidelberg. The train ride follows the river to BW from Heidelberg and is a very scenic one as well.

https://www.travelbook-magazine.com/travelbook-award/the-most-beautiful-old-town-in-germany-is-located-in

Hirschhorn is on this route as well:
https://img.fotocommunity.com/hirschhorn-am-neckar-a65c6bd3-3a61-4461-992a-1cba6afa5e9e.jpg?height=1080

ANOTHER QUESTION: Will your trip take place after July 10? The trains on the EAST bank of the Rhine (the Rüdesheim side) will be impacted heavily by railway construction projects from the 10th on:

https://rheingaulinie.de/files/rheingaulinie/files/Fahrplan/fahrplan.png

The St. Goar/Bacharach/Bingen railway, which I suggeted above for your Rhine outing, will not be affected.

I appreciate the info on the trains- our trip is in September so good to know.
Still looking at hotels, etc. In Heidelberg. Sounds like Mannheim might not be the ideal place to stay for 5 nights but my wife's mom & some cousins are from there. Any suggestions on places to stay somewhere close by that area? Thanks!

...our trip is in September so good to know.

That east-bank railway will be under construction throughout September as well. Stick to the Mainz > Bingen > Bacharach > St Goar side of the river for train travel and sightseeing.

Mannheim might not be the ideal place to stay for 5 nights but my wife's mom & some cousins are from there. Any suggestions on places to stay somewhere close by that area?

Heidelberg is a somewhat more typical tourist destination, but there is nothing wrong with staying in Mannheim if you have a good reason to be there. Mannheim is a common place for trade fairs, business conventions, etc., so you'll have no problem finding modern accommodations with all the conveniences and places like Best Western, Holiday Inn, Hilton, etc. that you are probably already familiar with. There are also locally-owned hotels that compete with these chains and offer similar services. Fortunately for you, many of these places lie near the main train station in Mannheim, which will make getting to and from your hotel most convenient when you arrive and when you head out to the Rhine Valley, to Heidelberg, or to other destinations.

We have no idea exactly what you are looking for in a hotel. And I'm sure Rick has nothing to say about Mannheim... and I haven't stayed in Mannheim in the last decade or so... There's a recent thread here started by frequent-poster mardee (someone worth paying attention to IMHO!) which puts the Premiere Inn chain in a very positive light - and there is a Premiere in Mannheim near the station. For fun I checked a random 3-night stay there in September... good reviews and $321 total for a double with breakfast? Sounds like something I would book as well.

https://community.ricksteves.com/travel-forum/general-europe/finding-your-heart-s-desire-the-perfect-hotel-chain

So maybe you book just 3-4 nights in Mannheim - whatever you need there - and the remainder in Heidelberg?

As the first poster, me.crewe pointed out, Heidelberg is only 12 to 18 minutes from Mannheim by S-Bahn and regional trains which are very frequent, like every 10 minutes in the morning.

Here are my thoughts on splitting your time between the two places, fwiw.

Heidelberg > Mannheim: 12-18 minutes from the main (Hbf) station, not a lot if minutes... but staying in the old-town zone, which is more atmospheric and often what visitors choose to do, means that you'll use the Altstadt station... which makes this trip more like 24-30 minutes.

Heidelberg Altstadt > Bingen / Rhine Valley outing:

• The quickest routing takes you via Mannheim. And there's a change of train which adds even more time when you start in H'berg. By starting from Mannheim instead, you will have just one change of train and will likely save around 60 minutes total on the round-trip.

• Mannheim and the Rhine Valley lie in the same "Land", so the 38-Euro Laender-Ticket (day pass) for two will be cheaper than the 63-Euro QdL-Ticket for two (day pass for all of Germany, the one-ticket solution if starting from Heidelberg.)

• There's something to be said for splitting your time between two places instead of spending it all in just one. These are two quite distinct places and you will see two sides of Germany for sure.