I think a party at the park sounds great. If you're trying to avoid nap times, I suggest 3:00 - 6:00 and do the food at around 4:00 - 4:30.
One thing you said caught me - the pinata. I know EVERYONE does them, but it just makes me so nervous having kids who are blindfolded swinging around a baseball bat. I never have them and I never watch when others have them. Just my own paranoia!
Also, drinks for the adults? Not alcoholic drinks, right? It is a kids party and you don't want to send kids home in cars when parents have been drinking. (Even beer and wine are alcohol)
I would also do a pin the tale on the donkey game. Kids of all ages can and WILL want to play. It's amazing to see how many kids absolutely LOVE that game! You could also do a little treasure hunt. Get there early and hide some clues leading to a prize.
You'll have a great time! Have fun planning!