Go to the roof at night and watch the light show at the eiffel tower, if you're into that sort of thing.
This is where the Parisians go on a beautiful day. Bring a picknick and have it on one of the lawns with students from Sorbonne and other locals.
Come on, you gotta see at least 1 museum while in Paris. This is a good one - beautiful setting and grounds, in and out in an hour.
