[Gwathagor]

Don't plan on trying to see all of the Louvre, because it's too big. If you MUST see the Mona Lisa, go early and make it the very first thing you see.

The Sewer Tour is really interesting if you've read Les Miserables.

I agree with Lalaith: Sainte Chapelle is splendid. Smaller than Notre Dame, but better stained glass.

If you like military history, Les Invalides is the place to go. They also have the Tomb of Napoleon.

The Pantheon is worth seeing, as is the Cluny museum. Jardin des Plantes has a cool zoo.

The Buttes Chamont park is nice; it used to be a huge sewage dump until the 1860's, so it has lots of tunnels and crazy hills and a nice artificial lake.

I don't remember London as well. The Tower of London is obviously a must-see (sooo many weapons!), and if you get a chance to visit the British Museum of Natural History, look closely for my finger-prints on the Rosetta Stone. They're there. They're not supposed to be, but they're there.