I took my kids to The National Inventors Hall of Fame last week and they loved it! I was a little underwhelmed when we first came into the building, but my children, age 4 and 1.5 played happily for 2 hours straight.
There is a water table, awesome build-your-own house exhibit, and a children's nook with picture books, a sound machine, blocks, computers, etc.
I do think the admission is fairly high for the museum, but it seems like they do not get a lot of traffic, so I understand the higher price. Overall, a great place to take little ones for a low-key afternoon of fun.