I've lived in Paris for seven years and I always take visitors to the top of Printemps department store (behind the Opera house). The building that has beauty products on the ground floor. There is a great rooftop bar, that is free, have a glass of wine and see great views of the city (incl Sacre Coeur and Tour Eiffel) and unlike the roof on Lafayette the view is not obstructed!
A wonderful museum in an old building is the Musée Carnavalet in the 4e arrondisement (by St Paul metro); it is of the history of Paris, very interesting and not crowded!
If the weather is nice, take a visit to the Père Lachaise Cemetery; very beautiful, some very elaborate tombs.
Visiting the catacombs is really cool and if you can stand the stink (bring a perfume scented scarf; I actually gagged) check out the sewers tour (self-guided), it's very interesting.
If you go to Versailles, make sure you visit the Trianons and the Hamlet. And the Opera House is beautiful and worth it, esp if you don't have time to see Versailles.
Definitely do Giverney, but I'd skip the tour. It is very easy to get to Vernon (next to Giverney) by train and then there is a shuttle that takes you to Giverney (5 min) or you can walk. Get tickets online to Monet's home to avoid the queues. Take a quick stroll through the town, and head back to Vernon for another stroll or visit the chateau there.
I highly recommend a quick stop in Auvers-sur-Oise. Charming town on the banks of the Oise river. Van Gogh died here and painted here as well. The church he painted can be visited, the home of Dr Guichet with many works of art by several impressionists, there is a chateau, and the river (many Renoirs, Monets, etc were painted here), and more. A nice little day trip.