Determine where in Paris you want to stay, and then find an apt. in that area. There were some really cute apartments that interested me, but they were further (walking distance) from where I wanted to spend most of our time. You're never far from a Metro station in Paris, but if you want to be within walking distance of the major sights - be sure to stay close to the Seine! If you're going in the summer, you will be glad your apt. has air conditioning! (Not all do.) Also - note which floor the apt. is on, because many older buildings won't have elevators.
Buy Rick Steves' Paris book and the Paris Museum Pass - even if you're only spending as much time in the city as the ABD trip allows. You'll have some free time, and the museum pass will allow you to visit museums that aren't part of the tour while probably saving you money.
Oh yeah - get Rick Steves' London book, too!
