Lots of great suggestions here; brings back fond memories of my semester abroad (lived in London and visited lots of places in Europe).
Definitely check out Murano while in Venice. The glass made there is beautiful. And I had the best fried calamari I ever had from a street vendor in Venice. If time permits, take a trip "down south" to Naples, Pompeii, and the island of Capri. I remember waking up in the morning at the youth hostel in Naples and seeing Mount Vesuvius out the window -- wow, very cool! (Otherwise, yes, stay away from hostels. They are gross. We only did hostels to save money; otherwise, we stayed in "pensiones.")
Wherever you go, as long as you feel safe doing so, try to get up as early as possible and just walk around the city. Obviously there will be fewer people; and as long as it's not overcast, the lighting will be great for pictures. I got up early the first morning I was in Rome and went for a walk -- didn't bother to take a map because I wanted to be surprised -- and I turned a corner and all of a sudden, there was the Coliseum, all golden from the early morning sun. It was awesome! Also in Rome, definitely make as many gelato stops as you can!!
We had a really fun time in Munich. You can visit the BMW Museum; drink humongous beers at the Hofbrauhaus; visit the Olympic stadium (although we were "asked to leave" after attempting to run around the track...ah, good old college days...); and see the giant glockenspiel do its thing on the hour in the town square.
To be honest, we were disappointed in Florence. Yes, we did it in a day, it's definitely possible to do; and we went to all the places mentioned above. Though the art is beautiful, we found the city to be quite dirty and the people unfriendly.
Of all places, beware of the train stations the most. Many low lifes hang out there. At the train station in Rome, a gang of young Gypsy kids tried to steal our bags, and it nearly became violent.
Paris -- yes, eat crepes! -- and croissants! -- and cheese and wine!! Can't remember what it's called, but be sure to go to the museum that has all the Impressionist art (Monet, Van Gogh, etc.). Go to the Louvre and see the Mona Lisa (it's surprisingly small) and then jog through the rest of the place (the Louvre is huge). Paris was where we encountered the most anti-American sentiment. I literally had some guy come up to me and say "Yankee, go home!"
Make sure you have Italian, Spanish, French, and German phrase books. People are much nicer when you attempt to speak their language. They don't care if you mangle it; they appreciate the effort, as opposed to being the stereotypical American who expects everyone to know English. Also, wherever possible, use your charge cards to buy things; charge card companies get better exchange rates when it comes time to pay the bill.
Sorry this was so long. Have a great time and be safe!