Nerd Heaven for me....
THE BOSTON MUSEUM OF SCIENCE!!! I could spend days in there. I've been to several exhibits on Ancient Egypt there (including an IMAX movie- if you've never seen one you need to go before you die!).
The Last two times I went down to Beantown it was with school groups and I always get pressured into skipping some of the things I like based on my chaperone or the group I'm in. For example: I love, love, love the natural (green wing) exhibits about wild life and ecology, but I think we spent all of 5 minutes in there last trip. I also like to just take time to see all of the hands-on exhibits, but everyone else I know just rushes through them.
The most recent Egypt exhibit I went to see was soooo amazing. They re-created a genuine burial chamber (with all the authentic heiroglyphs on the walls and everything) and I just stood in the middle and spun in a slow circle, but then my group came looking for me from three rooms down and I missed
the next two rooms trying to catch up.
I swear, once I get my license, I'm going to that museum for the day.
) and Rose...well you know what I'm talking about. I got to walk down the staircase because I was choosen out of the group of people we were with. 
It was amazing. They also have an outside scene on the decks, with the lounge chairs and with stars in the sky that are lite up. It's amazing, it makes you honestly feel like you're standing on board the Titanic the night it sank. There is also a Titanic exhibit in South Carolina in Myrtle Beach that I visit all the time. It's not as good, but I love it. 
