1st trip overseas and to Germany

Germany is one of the more pass friendly countries, but single ticket can still be a lot cheaper and in your case I suspect they are.

Welcome to the forum.

A "travel base" plan of attack that houses you in 3 different places and sends you out on day trips is a nice way to travel. Normally a plan like yours could be done fairly easily by pre-booking 2-3 major journeys - and then you would add on short local train journeys for your day outings from each base. In your case Regensburg really is just a local train trip. So maybe your major journeys look like one of these...

• Fly into FRA / Major Journey 1 to Munich (and use local transport to get to your 3rd base town, Regensburg, and back) / Fly out of MUC

• Fly into FRA / Major Journey 1 to Munich (and use local transport to get to your 3rd base town, Regensburg, and back) / Major Journey 2 back to Frankfurt / Fly out of FRA

Your major journeys should be fairly cheap right now since your travel dates are pretty far out. Buy them as soon as you can commit for the cheapest price.

The local day trips, to be bought locally, including your Frankfurt > Heidelberg > Frankfurt outing, can often be done inexpensively by day pass (like the Bayern Ticket) - normally at a cost of around €30 - €50 max per day. A day pass will typically cover local trams, buses and subways you might need as well - a big advantage over the rail pass (which covers just your trains.)

Unless you have some unusual day outings that require a Regensburg base, I would actually recommend Nuremberg as a train base town instead - the day trips from N'berg are numerous and excellent and typically cost just €20/day/couple.

https://vgn.de/en/tickets/all-day-ticket-plus/

(With Nuremberg and Munich as base towns, you don't need to add any pre-booked major journeys - N'berg > Munich can be done by local train / day pass as well.)

Regensburg can be a day trip from either Munich or from Nuremberg.

If you had 3-4-5 travel bases spread out across the country, it could be worth looking into a "flexi" German rail pass for those major journeys (and possibly for some or all of the day trips too since the cost per day goes down the more days you buy.) The twin pass for a couple can be a good bargain in this situation. There is no rush at all to buy the rail pass if you go that way. You can even buy it once you're in Frankfurt if you like. And in the past... DB has offered "autumn promotion" prices for some rail passes for October travel. So just hold off entirely for now on any GRP purchase.

I don't believe Füssen is worthwhile as a day trip from Munich. If like every tourist, you're convinced you need to visit Neuschwanstein, make it an overnight stop for 1-2 nights - on a day outing from Munich, you will have 5 hrs. of train/bus transport, a couple of hours of walking, and little/no time for anything else in the area.

It's a common assumption that a railroad pass saves money. That's getting the process the wrong way round, as you seem realize. Researching the point-to-point tickets first gives you the grounds for comparison. For local tickets, advance purchase often gives the only advantage of not lining up at a ticket booth. Inter-city tickets may be cheaper bought early and could include the mandatory seat reservation that passholders have to buy separately.
The Bahn site is a very good source of both information and sales; you may already have it. https://www.bahn.com/en/view/index.shtml

Even wider perspectives can always be gained at www.seat61.com

Flying into one city and home from another is also efficient, so long as you use a multi-destination search function.

With 21 days... and with Frankfurt on your list... it's odd to me that you have left the Middle Rhine Valley out of your plans for outings. The scenic part of the Middle Rhine, stuffed with castles, old-world towns, and vineyards, is a great place to be in the fall (wine/harvest festivals) and is only 1-2 hours from Frankfurt; there's a lot there, so spend a couple of nights there and you will have time for some river cruising, for touring some real castles (like Rheinfels and Marksburg) and more.

Map w/ FRA airport location shown

Castles

Boppard

Thanks for all of the info! I’m probably making more out of this than necessary but since we’ve not done this before is seems overwhelming.

So what I think I understand from your comments.

1. Buy the big tickets (ICE) to the major cities, Munich, Frankfort and Nuremburg prior to trip.
2. Buy day trips To outlying towns from base towns on the day we’re headed to those particular towns. (Regional towns?)

We definitely are including Rhine valley....I just named a few.

Thank you in advance

Buy the big tickets (ICE) to the major cities, Munich, Frankfort and
Nuremburg prior to trip. Buy day trips To outlying towns from base
towns on the day we’re headed to those particular towns. (Regional
towns?)

Yes, that's the BASIC idea, liebet.

"Day passes" is a pretty big category. There is a national day pass, then there are "Länder" (state) day passes, and then there are transit authority day passes. You'll be on the Rhine. So let's say you are staying a few days in Rüdesheim (a popular and touristy place not far from Frankfurt.) Which day pass is best for your train journey depends on your destination.

• Going into Frankfurt or Mainz for the day? An RMV (the local transit authority) day pass ("Tageskarte") is in order.

• To Cochem (Mosel River) you'd want a Länder Ticket for the state of Rheinland-Pfalz (Rheinland-Pfalz ticket.) Even though "ticket" is part of the name, it's a day pass.

• An extra-long day trip - to Stuttgart for example, which takes you outside the R-P travel zone - would require the national day pass for Germany (Quer durchs Land ticket) which is constrained only by the German borders.

All of these day passes put you on the regional trains (RE, RB, VIAS, and other designations) and forbid travel on long-distance trains (IC, ICE, EC, etc.) Most have a single-traveler ticket price and a small-group discount price for 2-5 people (or a price that goes up incrementally with the # of travelers.) Most (but not all) have a restriction on weekday travel before 9 am. Most transit authorities offer a day pass price for system-wide travel anywhere you like within the zone, the whole day long. But they also have cheaper prices for shorter distances.

Example from a Rüdesheim base: a day trip into Frankfurt and back for two adults on a mini-group ticket/day pass is €35 for the RMV ticket because Frankfurt is a "price-level 6" trip based on its distance from R'heim. If you were going to Hanau, a price-level 7 trip, you'd pay €45.50.

These local transit authority day passes can differ quite a bit. If you were to stay in St. Goar (Rhine,) the VRM transit authority for this area offers a system-wide "mini-group day pass" or "Minigruppenkarte" (valid all day Sat or Sun, on weekdays after 9 am) for €24. That's a big discount as the single traveler pays €21. But the single traveler can buy a 3-day pass for €42 (which is valid for three full days, irrespective of the day and hour.) Multi-day passes are somewhat unusual. In the NUREMBERG region (VGN) you can use a system-wide day pass (priced at €23/couple) at any hour of the day, any day of the week. Buy it for Saturday - and remember to keep it - you can use it on Sunday too.

Most smaller stations (like the Rhine towns) are not manned with agents - the day passes can be had at ticket machines - you input your destination, and the machines normally provide the options available at your chosen hour of travel. Read what this traveler to St. Goar (Rhine) says about using the machines.

GUEST TICKETS: These have become a "thing" offered by some local tourist information offices - Book in certain hotels, or in some cases ANY hotel in certain towns, and you can take local day trips for FREE for the duration of your stay with the free guest card you get upon registration. So you might not need to purchase a single day pass at all. BOPPARD (Rhine) is a great town with lots of room and dining options and a very good base town for train travelers. Any room or apartment will get you one of these cards. Details are here.

Several informed forum members here can help you figure out what you need. Come back once you know your day trip destinations.