I am so not the creative person. but here is how I did my girls' birthday party last weekend (their birthdays are in May and June, so we had a party for both of them. They have many of the same friends, anyway).

I invited kids and the parents (I had invited about 40 total, but 25 came). We had a cookout, which was just burgers and hotdogs, very inexpensive. I stated on the invitation no gifts. Don't they have enough stuff as it is? LOL! Also, told the kids to bring swimsuits and towels. I got a Slip N'Slide ($30 at Walmart) and the kids played ALL DAY in that, along with another hose with a nozzle attached to spray each other with. The kids were ages 2-12 had an absolute blast. The parents sat around talking and watching. I ordered a cake (1/2 sheet for $25, with both their names on it and a cheerleader) because I just cannot decorate a cake. I made homemade ice cream that morning and it was ready to go for the party.

We were going to give the kids a ride on the 4-wheeler, but they were so into the water, we never even mentioned it. The entire thing cost less than $100. And everyone, even the parents, had a good time.