Words can't explain the feeling you get when walking around inside Notre Dame. It's just humbling and so spiritual. Early in the morning seems to be the least crowded (as soon as it opens, I think around 8am). It takes a while to get into the Sainte Chapelle. You should get the museum pass. This will cut about 30 minutes off your wait for tickets to get in. You also have to wait 20 to 30 minutes to go through a security check (scanners, like at the airport). I HIGHLY suggest buying the museum pass, which I didn't buy this time. It gets you into all the main museums and a few of the things like St. Chapelle.