We have a extended family and friends who come a distance for birthdays, plus local friends, and I don't want hoardes of people climbing all over our little house, either. We're having this year's b-day party at the "Little Gym" where my son takes a tumbling class. We've done that for the last two years, also and it's worth every penny. It's hosted by two or three instructors (depending on the size of the group), there's an hour of theme-based class (you pick the theme) and half an hour in another room for cake and presents. Our location allows non-members to have parties there too (for an extra $25, I think). After the festivities "off-campus", family members and out-of-town guests come back to our house for pizza and salad. It's still a zoo, but it's maybe 15 adults/kids instead of 40 adults/kids.
We've also attended a couple of birthdays this summer that were held at a bouncy-house place - both were great. The kids wore themselves out jumping and the adults could stay a bit removed but still within sight of the kids in the party room.