Big Man just turned 6. We had a great party at an underappreciated museum, a place full of obscure machines and items previously thought to be too quotidian to attract notice. I love that kind of thing - the reconstructed rooms used to be my favorite part of the National Museum of American History, one of the few museums I never had anything to do with professionally. Hence, I haven't been there since I was a kid, and now they're renovating the joint, so those rooms are probably history (ooo, meta!).
At our party we had cake, we encountered a wacky old docent who demonstrated the machines and let the kids hoist cargo, there was a fairly successful engineering activity involving masking tape and marbles.
But I think my favorite part was the ride home. We packed five boys, ages 4 to 7, into the minivan and listened to them tell each other lies and crack each other up all the way home.