Planning a Party

So i'm planning a birthday party for my son. I'm getting a big pool, some pool toys etc.. i'm also thinking of having a snack table that includes popcorn machine, cotton candy machine and an ice cream/ snowcone maker... any suggestions for other cool and fun snack ideas are welcomed. Haven't quite decided on a theme yet, he chose from a long list and decided on Avengers, Justice League and Ninjago..so on that we're still undecided. We have a really big back yard and don't really want the pool to be the focal point, so we're setting up an area for volleyball, maybe soccer, and maybe a sac race... again, any suggestions are welcomed. It's gonna be an all day thing and alot of friends are coming... however, I haven't sent out invites yet but we've been talking to some parents. What should the invite say really? I know I wanna include water and sports in it..I have never planned a big party, just intimate settings, so this is different.. plus I gotta cater for children and adults.