Another favourite place of mine. The Living Computer Museum in Seattle has a fantastic collection of computers, both large and small, going back to the days of valves. And, as the name implies, many of them are working machines, including some mainframes.
I first went there a year ago, and I really wanted to go back again. So we did. They were having a grand opening of some new exhibits, and it was awesome. They now have robots, cubelets, embedded stuff, they have a full size computer controlled car and of course all the computers that I know and love. If you are in Seattle you really, really should visit. I'd go every week.