The website you got from Steve is the same one I found. Really good information on there. You can get to most of the towns by regional trains. For travel, your cheapest bet is to either use a Laender ticket, a Happy Weekend ticket or a Quer Durch Deutschland ticket. Depends on if it is the weekend or if you have anyone traveling with you. Or check the regular ticket prices between the various cities on the Deutsche Bahn website.
From Frankfurt to Hanau, you can take the S-bahn. Trains will go to Steinau, then Schluechtern, then perhaps go onto Marburg, which is a great university town. Your best bet after that it to travel to Kassel, the Goettingen, the on to Hameln, ending in Bremen. This should pretty much use up your 5 days. There are so many smaller towns along your route, but you really don't have time for them.
Looks like a fun trip. I hope you will come back with a trip report for us.