Hi K.,
Our son's first birthday was huge, too--I think I sent out 50 invitations (which would have been about 100 people if everyone came--which they didn't, thank goodness). We figured the first birthday was a good time for extended family and friends and even some work people to see our boy. We have not repeated the experience with the later birthdays--and yes, like you, our son was the only grandson, etc.
Here's what we did:
1. We hired a jumpy for the all of the kids--this worked great for our son who could crawl/bounce in it as the one year-old and the relatives' and friends' kids up to the age of 13. Jumpies are great for everyone! (I did not do games or anything else--the party was adult-heavy and people were happy to talk and eat).
2. For food, we went to Costco and bought many trays--shrimp, fresh fruit, veggies, mini wrap sandwiches, cold cuts and cheese, etc. We also bought many frozen appetizer thingies which were easy to heat and serve on platters. The only things I made were an enormous pot of baked beans, a jello molded salad, and a broccoli salad--all made ahead of time. We set it up buffet-style and let people graze as they wished.
3. Paper plates and plastic cutlery are your friend!!!
4. For party favors, Costco again--two boxes of chocolate bars that I glued a note to that said--"Thanks for coming to my first birthday!".
5. We filled ice chests with bottles of water, diet Coke, 7up, juice boxes, and beer. The water and diet Coke were in the most demand. We directed people to serve themselves.
6. We ordered an entire sheet cake for dessert. No ice cream--too time consuming to serve to so many guests. There was plenty of cake left over.
7. We did not open gifts during the party--way too many people to see and participate, way too time consuming, and our son was bugged out by the end of the party anyway--so many people. I also asked people not to bring gifts (again, the only grandson, etc. thing)--which most people ignored, but instead they brought small items, which worked out fine.
We also had a friend who had her son's first birthday catered--Mexican food--that was not too expensive and worked great since she could just chat with her guests (which I didn't get to do much of, even though a lot of the food was pre-done and self-serve--i was still locked in host-mode, which was fine but I didn't get to spend much social time with the guests).
And, we just had a friend's daughter turn one who did not have a party--just cake at home with her grandparents--so, really, there's no one right way to do a first birthday.
8. Ask your family to help--no matter what--especially get someone to run the video camera and someone else to take still pictures--we should have done more of this, since my husband and I were too busy to do much of this.
:-) D.
P.S. I love the ideas that other moms had about a potluck--you could easily potluck my food ideas above--just make sure you don't make someone responsible for a crucial item who is always late to events or who leaves early... :-)