make sure your trip involves a train ride through Switzerland/Austria. Switzerland is a great place to visit too, just not as much of the cultural/art stuff you'd get in countries like Italy.
As for Italy: Venice-Florence-Rome. In that order. If you have some time, do a hike through the cinque terre in north western Tuscany. It will be packed wtih tourists during the summer...but it's Italy.
the South of France is a great place to relax in the summer. There is a large international fireworks festival in Cannes during the summer; but I think it's after the dates you allotted. France is great, but Parisians suck (most Americans' view of the French is mistaken for Parisians). If you get a chance, head into Czech Republic, Croatia, or some of the Greek Islands--the Cyclades [sic?]. If you want to hit Spain, and esp Barcelona (excuse me: <lisp>Barthelona<lisp> ), be sure to staple all your gear to your back and guard your asshole. Someone will try to steal your ******, and there's a good chance something far more devious could happen......
Get a Eurail pass. Under no circumstances, don't get into one of those "30 towns in 30 days" pre-paid student tours. They're totally worthless. You don't see a thing, and you're too rushed/hurried to have any real fun, or learn a damn thing. Best to pick a few spots and spend the majority of the time exploring the local environs....like spending a week or so in Florence, and making day trips from there to Siena, San Gimignano, Arezzo, etc.