K.P.
Don't worry too much about decorations, just pick a "color theme" and get the papergoods and some balloons. Just a word of advice, unless you feel compelled to have paper tablecloths, avoid them outside! Tough to keep them on the tables and when someone spills, you toss them anyway!
Games/ Races to play in groups:
- water balloon toss
- egg on a spoon relay race
- fill the water jug relay race
- 3 legged race
- bubble gum blowing contest