Versailles: If you are staying more than 4 days, I would highly recommend a day trip to Versailles. It's like nothing else I've seen. It's such an experience. You can get a guided tour but we got the hand held audio set and went at our own pace. The gardens and grounds are beautiful and it's free with your Paris Museum Pass. You will need to take a 45 minute train to Versailles from Paris so this is a full day trip. Be sure to check out the Hameau de la Reine, Marie Antoinette's hobby farm, built as a retreat from her life as a queen. It's bizarre and intriguing. There are a few restaurants and snack stands on the ground so you don't have to leave for lunch.