If you don't mind staying a little later, I believe many of the bus trips leave in the afternoon and the evenings are much less crowded.... its open till 9:30 it says.

Go at dinner time, like 5:30. The afternoon visitors are on their way out and the evening visitors haven't arrived yet.

I've been there right at opening in the morning and the crowd was light. The afternoons are very crowded, but I like to stay all day and pace myself around, crowds or not.

I went yesterday (opening day, obviously) and it was MOBBED by 9:30. We got there at around 8:30 and it wasn't so horrible but some displays were already so crowded that I told my boyfriend not to bother and we moved on. I know this was probably because it was the first day.

I would probably suggest around dinner time. Everyone seems to want to come after lunch and it gets hectic (no one wants to move, people get in the way, awkward plants hit you in the face)...no fun. Then again, I despise crowds.