A couple of points.
Munich especially and Bavaria in general is quite expensive, more iMO than further north. Also, for learners, the German in southern Germany can be quite hard to pick up/understand, quite different frm the standard German taught in schools. Although all of Germany has dialects, you may find the area round Cologne /Bonn/other parts of north has more 'standard' speech. .
I love Bremen BTW, used to live near there, in a previous life, really want to go back. I like some of the smaller cities - I've taken my teenagers in alst few years to places like Bonn and Magdeburg. M's yh very near train station BTW. Every German city has at least one museum, if your boys like that. Both ahve very nice youth hostels - excellent breakfasts!! Magdeburg has interesting history, though not loads to do, but we used it as base for other day trips. eg Berlin is within easy train journey. We used train a lot, indeed went by boat from harwich as I don't like flying. BUT Magdeburg is between airports like Hanover and Berlin. Also leipzig - we did day trip there from Magdeburg too.
Bonn (also near airport) is great base for other places, as welllas lots of historical/cultural stuff in its own right, like Cologne, and also the spectacular views of Rhine valley. Lots of lovely little towns/villages to visit. Bonn YH very popular though, as lots of German school groups visit. Bonn has hugely significant ofr post 1945 German history, obviously. And is part of curriculum. Another lovely hostel is Stuttgart - another useful base and a lovely, smallish city in own right.
We've had some lovely weather in Germany in March, T shirt wearing weather. Quite a contrast too when we got abck to Scotland, to inches of snow. That was March 2010, maybe 2009, i think.
Have dug out my Rough guide to Germany this AM. I can feel another 'trip plan' coming over me. Swoon, swoon. I love Germany. I've been ill (and poor!)and not got to travel as much as I'd like. If you ahven't got a book like RG, i think you might find find it useful.
I could go on. What 'sort' of teemagers are your boys? My boys a bit 'geeky' so history and museums v popular here. But germany huge and has something for all, methinks.
Gute Fahrt!!