Travel is so subjective, something might be the high point of one person’s visit, while another person will be totaly indifferent to it. Bearing that in mind . . .
Get yourself a small European phrasebook. Mine has just about everything you’d want to say in six languages. Learn a few basic phrases just to let people know you’re trying. Usually they’ll continue the conversation in English. The exception is Dutch: it’s an impossible language to figure out on your own, and they’d rather you speak English anyway.
Don’t spend the whole trip taking pictures of each other in front of famous sites. It’s amazing how many people do this, as if they have to prove they were actually there. Instead, take pictures of things that you want to remember: people you meet, funny things you encounter, interesting scenes, street performers, etc.
Water: You’ll get very thirsty walking around, and there are damn few water fountains in Europe. Carry bottles of water, and refill them each morning from whereever you’re staying.
London: My favorite thing in London was the half-day trip to see Stonehenge, though others have been disappointed. Also, the British Museum has some neat things in it. The food in London is much more varied than it used to be; and of course, the many, many pubs.
Paris: I can’t go to Paris without visiting the d’Orsay Museum, which has art from the impressionist and art nouveau periods. Especially the top floor (a room of van Goghs, a room of Monets, etc.). And the building itself used to be a train station, and is gorgeous. Paris has the best food anywhere, but can be pricey. Eat lots of baguettes.
Rome: I spent almost an entire day exploring the Coloseum, but that’s just me. And you’ll have to learn a whole new way of crossing streets in Rome; the traffic moves fast, and never seems to stop.
Florence: If you see the statue of David outside, that’s not the real one. The real one is inside and very worth seeing, especially since it’s recently been cleaned.
Venice: try to get to Venice, even for a day or two. It’s a magical city and so much fun just walking around and getting lost.
Vienna: What I remember most from Vienna was seeing the original paintings of Gustav Klimt; no reproductions can do justice to them. Also, the architecture from the Jungstil movement. And if you’re at all interested in classical music, a whole bunch of major composers lived (and are buried) there. And Vienna has the best food outside of Paris.