Think of what you would find at a carnival - Now scale it down.
For examples: Bobbing for apples, floating ducks with #'s on the bottom that = gifts, you can rent cotton candy machines, popcorn machines, etc.
If you are like me and can't really afford to spend $500 on a birthday party you can scale it down even more.
You could make your own beanbags, get a board and cut holes into it - now you've got a beanbag toss game. Make a homemade pinata. etc. etc. etc.
You can do something that cracks me up everytime, where all the kids line up and the object is that they each have to eat 2 saltine crackers and sing Happy Birthday within 2 minutes - The funny part is the crackers of course are hard to completely swallow without a drink so as they sing it's 1 hard to understand and 2 they all have cracker coming out while singing. It's one of those gross, funny things.
The biggest thing you will need is PEOPLE - You will need adults willing to "man" each "station" for you. Most parents are more than happy to because they know how much fun the kids are going to have.