Man, so many choices! Spring could still be kind of rainy in the alps, but if you want to risk it, the sheer beauty of that place is so rewarding. I think I've already told you how much the Lauterbrunnen Valley means to me...
See what I mean?
But given the time of year your're going and the number of days you'll have, I suggest Berlin. It's one of the most amazing European cities... has a TON of history associated with it, yet is one of the most progressive cities in Europe. See the Berlin wall (the tiny bit that remains), the Reichstag, Brandenberg Gate, Checkpoint Charlie, Wilhelmstraße, Bebelplatz, Potsdamer Platz, Zoo Station, the TV Tower, I could go on and on. Check out these
pics.
If you want to see Bavaria, can't go wrong with Munich either... it's quite a contrast to Berlin.
Most Germans know some english but many are reluctant to speak it. Contrast this with Austria, where most people know some english and most are very willing to speak to you in it. Salzburg is a BEAUTIFUL city... one of my favorites. Tons of culture and things to do and see.
I wouldn't try to cram Italy into such a short trip... you will eat up an entire day (or night) on the train just getting there and back (I assume you're using the rail and not flying). Although Italy is definitely worth seeing, save it (and greece and spain) for a different trip.
I can highly recommend Switzerland, Germany, and Austria. Anywhere farther would eat up too much time traveling, IMHO.
Man I'm getting jealous... MUST plan another Europe trip.
l2c