We went to both the Washington Side and the Oregon Side. While the Oregon side has the hatchery (which is worth seeing), the Washington Side is MUCH better for touring the inner workings of the Dam itself. Built in the 1970's, this dam was set up much better for visitors. The Oregon side, built in 1936, is more historic, but you don't get to tour the inside. Wait til you see the inside of the Washington side dam, it's HUGE. They take you right up to the generators too.
We went on a nice day in February, and there were three people total on the tour. I'm told it's much busier in the summer. There's a very nice fish ladder exhibit as well.
Best of all -- IT"S FREE!