We did the same as KC, had an open house. We just had lots of food (one little girl ate FOUR cupcakes....then she CRASHED in the car on the way home) and margaritas for the adults! It was a blast, even though ds was rather cranky, wouldn't let us put him down, and actually FELL ASLEEP (this is a kid who HATES to miss out on the fun) halfway through (when he woke up, only two people were still there, so he was much happier. I think he is like dh and I...doesn't like a lot of people).
It was low-key, low stress, and fun!
We didn't do cake and stuff (we had cake, but not for the b-day boy) or open presents...he indicated that he wouldn't be cool with being the center of attention like that (although we did sing, at the request of cupcake girl, and he seemed to enjoy that).