Hey!
We were blessed to have a nice back yard for my son's first birthday. So we just blew up a $10 kiddie pool and my husband made a home-made slip and slide (with tarp we already had and baby oil ) and we had a fun hose/sprinkler thing. We still had room for some tables for the adults under the trees. It was great! There were kids of all ages and adults numbering about 40 or 50 total. We had the birthday at 2:00 pm so it was sfter lunch and before dinner. So I just made a cake and we had chips and brownies and snacks like that. It was a great time of water fun for the kids and fellowship for the adults. Our family and close friends offered to bring some snacks or drinks so that really helped us, too.
For my daughter's first birthday we moved and now had a parking lot for a yard but a larger house. So we still ended up having around 50 people and kids of all ages. So we bought the cheapest burgers and hotdogs we could find (at costco) and grilled out while the adults just socialized and the kids played with toys. The kids even played baseball outside in the parking lot. (Before you think I'm an idiot I should tell you that the parking lot surrounding our house is actually the parking lot for our church, so there was no traffic) The party was at like 6:30 - and went on until whenever the kids crashed. But it wasn't too expensive. we only bought the burgers and stuff.
Our parties are mostly just family and friends that are like family - so there is no formality. Just fun, food and fellowship. Good luck with ideas.
In fact my son is about to turn 3 and we are party planning. We are going to have it at my mom's house because she has a pool. And we are going to do it at around 3:30 or 4:00pm. until about 6:00. I think I am going to bake a bunch of cupcakes for the kids to decorate themselves and maybe do a craft (like paint masks made of paper plates and stick them on popscicle sticks) My friend was also saying instead of a gift bag (which I never do) get a pack of crayons and maybe a coloring book from the dollar store. Just tie with a ribbon and hand out at the door. She said they are usually 2 for $1. But thats a good idea for any age child, boy or girl. We are probably going to have about 50 people there as well , but only about 15 kids (of all ages). We may do burgers again or just hearty snacks because of the time. Also little ceaser's pizza has the $5 pizza. You could do as cheap food.
Anyway I hope SOMETHING from this helps you. =)