There are other science related attractions in Pittsburgh, but the Carnegie Science Center is the biggest and best on in the city. There are 3 floors of exhibits (not counting the basement which houses a small cafeteria). The exhibits change throughout the year, so if you visited last year, there is sure to be something new now. Admission is a little expensive (in my opinion). However, it's worth going to at least once to check it out.
There is also a planetarium and an omnimax theater, but I think that you have to pay extra for those (on top of general admission) if you want to use them.