Let me throw my zwei pfennig in here...

Rosetta Stone is a pretty good way to learn German, short of actually practicing it with live German speakers. I'm using it now to brush up on my German and am finding it to be a nice challenge. I disagree with the assertions that German is an extremely difficult language to learn. When you have a basic understanding of the mechanics of it, it actually makes more sense than English. In any case, there are quite a few similarities. Don't get fixated on the die, das, der (try to learn them, but don't lose sleep over mixing them up from time to time). Practice writing in German and visit German websites or watch German TV and you will be surprised what you start to pick up. If you have access to children's books in German, that is also helpful.

Heidelberg and Mannheim are in the southern part of Germany, and both also have higher concentrations of US troops, so you may find that English is spoken with some regularity. Do your best to speak German, but don't be afraid to fall back on your English if you get in over your head. I've never had an Germans get angry with me for screwing up a phrase, and most seem to appreciate the attempt to speak the language, properly or not. Maybe you can match up with someone who will help you with your German if you help them with their English.

Both of these towns are located on main train lines that will get you pretty much anywhere in Germany.