Fat Tire Bike Tours
Paris-Berlin-Barcelona
We took the Segway tour in Paris (sounds ghey, but it was actually a blast) and the bike tour in Barcelona was excellent, a must do if you go there. Have not done it in Berlin, but I can imagine it would be a good way to see some of the sites.
Paris - I could write a book on it. You've already gotten some good advice. If you can pickup a copy of Walking Tours of Paris, it's very good. My 4 day tour of Paris would be something like:
Day 1 - some easy tour in the morning (like on a Segway!) to help work out the jet lag. Have a nap. Go up to Sacre Couer and wander around Montmartre ending at The Moulin Rouge or pick a walk out of your walking tour book. If you don't do a bike or similar tour then go see the Eiffel Tour (get off at the Trocedero metro stop, it has the best view of the tower) and walk down to Les Invalides then do the above.
Day 2 - Do either The Louvre or Musee D'Orsey. Note they close one day a week so make sure you don't get caught out. Pompidou isn't a big draw in my books, but if you're a modern fan go for it. I'd suggest the Musee Rodin for the great sculptures. It's also free one day a week as I think some of the other museums are too. Go up the Arch de Triumphe at night and see the lights and then wander over to the Eiffel Tower for the same.
Day 3 - The other museum(s) you want to hit, a walking tour, and the arch/tower if you didn't go the night before.
Day 4 - Do the island visiting Notre Dame (the outside is fine) and the government buildings and such around it. Walk down the river on the side opposite Notre Dame (Rive gauche) and check out some of the street sellers and get a real sense of the city. Walk into the 5th (Parthenon) and 6th (Luxembourg) Arrondissements to get a sense of city life.
For food nothing is as Parisienne as buying a baguette from a corner patisserie and just ripping off pieces of it as you wander the streets.
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Barcelona is a fantastic city, I highly recommend.
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There is nothing in Frankfurt worth seeing. It was pretty much bombed to nothing in WWII and is just a modern city now. If you want old Germany I would pick Heidelburg or Nuremburg or if you want to be closer to Frankfurt, pick Mainz. All have the old medieval charm you're looking for. Nuremburg also has some of the war history you might be after.
Berlin was awesome. Tons to see and tons of WWII history. Sadly the parts of the wall that were still up when I was there are no longer around other than a couple of sections turned into an outdoor art gallery.
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I liked Venice, but maybe just because it flooded when we were there so it was rather memorable sloshing down the street in my shorts on the way to buy some rubber boots. It's worth perhaps a day and as others have said, spend more time in Florence or down south.
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Switzerland is pretty, enjoy the skiing.
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Back in my youth the future Mrs Furnace and I did a month tour of Europe. We did it in two loops from Paris as our home. Both loops were about two weeks:
Paris-Antibes-Geneva-Heidelberg-Salzburg-Venice-Florence-(night train)-Paris
Paris-Antwerp-Brugge-Amsterdam-Aachen-Berlin-(night train)-Paris
Looking back I wouldn't cut out a thing (other than Aachen, we stopped there to see friends...but did get a ride to Berlin and a free place to stay!) it was a good intro to Europe and left us wanting to see the rest in other trips. We did spend about 3 weeks after that month in Paris and area so Paris site seeing isn't included in that month.
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I have pics of some of these location on
my site. The above backpacking tour was way back in the 90s before anyone dreamed of digital and I haven't scanned my pics so you won't see anything from that era sadly but there are bunches of shots of Paris and places like Barcelona and Frankfurt.
The last advice I have is see as much as you can but also stop and try to find some "real Europe" in behind all those postcards, plastic souvenirs, and commemorative coins. Also start saving for your next trip because as soon as you get back, you'll want to go again.
Have fun.