Train and bus travel within the Black Forest is free throughout your stay if you stay in any of 130 or so villages:

http://www.blackforest-tourism.com/konus

Regional daypasses for train travel in the Rhine/Mosel region: good for up to 5 people along the Rhine to Bonn in the north, Karlsruhe in the south (which is almost to the Black Forest) and for travel to the west along the Mosel and elsewhere to the French, Luxembourg, and Belgian borders:

http://www.vrminfo.de/en/tickets-and-fares/ticket-offers/rheinland-pfalz-ticket/

Thank you Russ! WIll look into the links above!

Commenting on the last part of your itinerary ...

I've been to all the sites you mentioned - Rhine valley, Neuschwanstein and a little bit of the Black Forest.

Neuschwanstein is much cloaser to Munich, not Frankfurt (and it's still quite a ways from Munich). It's worth visiting, but if you used Frankfurt as a base you would spend a lot of time in transit - time which might be better used sightseeing.

I would pick the Rhine Valley any day over the Black Forest. Admittedly, I spent only 1-2 days in the Black Forest, compared to about a week in the Rhine and Mosel valleys. I liked the German Clock Museum in the Black Forest, and I understand Baden-Baden is a nice town, but some of the sights were disappointing, especially the waterfall described as the tallest in Germany (actually a series of very unimpressive cascades).

If you like castles you will especially like the Rhine valley. There are all different flavors of castles - large and small, restored and ruins. If you have time you might also consider the Mosel Valley - the river is much calmer than the Rhine with little commercial traffic. Of all the castles along the Mosel, Burg Eltz (which is actually located in the woods a few miles west of the river itself), was my favorite. Trier, at the southern end of the Mosel valley, is an old Roman town. The Porta Nigra in Trier (old Roman town gate) is impressive.

I drove along the Rhine and Mosel valleys. There are trains along the Rhine valley; I'm not sure about rail connections along the Mosel.

Trains are fantastic in Germany and on mainlines in Poland as well - in Germany two trains an hour or more going between most main cities. anyways if taking as many trains as seems in OP (if not renting car - car could be hard to take into Poland perhaps - check on that) - then consider the Germany-Poland railpass that lets you hop on any train anytime in those two countries and is also fully valid on K-D boats on The Rhine River (www.k-d.com) - fully flexible and fully flexible tickets can cost a fortune - a couple of long trips could make the pass pay off - if not wanting flexibility and willing to lock yourself into a non-changeable non-refundable train weeks in advance you can try to get the limited in number 29-39 euro fares which however often sell out weeks early.

Anyways some great sites to check to learn oodles about trains in Germany and Poland IMO; www.seat61.com; -http://www.budgeteuropetravel.com/id9.html and www.ricksteves.com.

I agree with just_wandering: focus on Rhine and Moselle valleys and drop the Black Forest (pretty dull) and the castles in Bavaria (too far away).

On the Rhine valley, apart from the numerous castles, there are also small wonderful villages like Rudesheim, Eltville, Bingen, Boppard. The boat trip on the Rhine is also very pleasant (we have just taken one from Koblenz to the Lorelei statue).

Then, on the Moselle, start from Koblenz, see Burg Eltz, but also Cochem and Bernkastel-Kues, before going to Trier (a must see) and Luxembourg (just 45 km away from Trier).

We have been to most of these places and you can see photos / tips by using the search function on our blog: http://in-luxembourg.blogspot.com.

How are you getting to Bremen? Do factor in travelling time. It's quite a way from Frankfurt, so that you could take a plane there, so as to not lose too much time. In the train it will take a good 4 -5 hours, so that's pretty much a day gone, if you haven't planned for it. There are probably enough things to do for 2 days in and around Bremen, and maybe your friends will take you around. They will hopefully take you to see the Roland statue, the town hall, the Schnoor district (tiny medieval district), the big windmill (now a cafe) and maybe on the second day you could go to Worpswede, which is a little artists' colony, which has beautiful old farmhouses in the north German style. That would give you a nice contrast with the beautiful buildings in Bremen (many stately 19th century houses, ornate medieval town hall).

Lavandula

Oh, and if you visit the town hall, be sure to see the statue of the Four Musicians (Die Bremer Stadtmusikanten). This is a rooster, a cat, a dog and a donkey on each other's backs, after the story by the Brothers Grimm. It's said that if you grab the donkey by the front hooves you will return to Bremen again.

Lavandula