Tap water w/ice in the cooler is much more economical than buying bottled water and you won't go crazy when you find half-used bottles all over the place later on. 9oz cups are a good size w/ a marker for putting their name on the cup. You could also get some juice boxes at Sam's or Costco.
A fun game we've played that is good for kids about 5 & up. Get some old adult size t-shirts and the night before the party, get them soaking wet (ring out excess water) and wadd them up and put them in the freezer over night. Give each kid a frozen t-shirt and the first one to get it pulled apart and put on their body wins. Enjoy your party!