Hi Momma-
I really think you have it down as far as group games. You could give the kids party bags with kazoos and things that they can play with. Since you will be at the park, sit near the swings or the play set so kids and parents can eat, play, pinata, eat, play..... Really, the less you have planned besides lunch, pinata, coloring, and happy birthday song with cake, the more running around the kids will do and you don't feel like you have to organize/ fill the entire block of time. My sister in law did this for her little one, and there were kids ages 9 months to 8 years old who had a blast during the 2 or so hours they were there. If you really want to spice it up, do some parent kid races, like the old picnic days- or a water balloon toss. The really little ones will get a kick out of things getting sprayed with water and you really can't get hurt. Just warn people ahead of time about the fact they might get wet.
Have a blast!!! These little one parties are really a hoot, especially when you start watching all the kids and how they entertain themselves. =)
-E. M
I just had a thought about hula hoops. Have a few toys out, like hula hoops, etc., and let them do their thing.