Sounds like a blast! Yes, If you can get a DJ, that will be a huge help. The DJ can also be the announcer if you give him/her instructions.
A scavenger hunt is great and if your daughter thinks they will play games, go for it. You could do Hula Hoop.. Who can Hula Hoop the longest. Limbo, How low can they go? The most flexible kid. The largest hands, the smallest hands. Who can hop on one foot the longest? Have a Volleyball net and Volleyball available. Frisbees are fun. Most of the times, the kids we know just like to sit around and laugh and talk.. and take photos of each other..
I of course love an age appropriate pinata. Since it is hot outside, nothing that will melt should go inside.. I would suggest candy, gum, mints, key chains, lottery tickets, quarters anything else you find at Walgreens that teenagers enjoy. Make a budget for the pinata, or you can end up spending way too much..
Make sure you have a ton of ice and Ice water along with whatever other drinks you are providing. Purchase plastic cups and have the kids write their names on them with Sharpies, so they can keep up with them. We ordered custom plastic stadium cups for our daughters big party last year and the kids loved them.
An assortment of sunscreens and bug spray will also be welcomed.