I'm a summer birthday myself & so is my younger daughter. I must say we have it pretty easy when it comes to planning a summer party. There is so much more you can do outdoors than we can in (mainly because we are in an apartment)
Water Balloon fight:
Fill up a ton of water balloons and store them in an extra cooler the day before the party (that or get up really early and while drinking coffee start filling them up) It might be good if you have a lot of people coming to have two coolers reserved for the balloons. When it's time for the fight, go over whatever rules necessary & let the kids run wild. This is also fun is any of the kids have a water gun.
Red light, green light
Object: To be the first kid to the "traffic light"
All the children stand at one end of the backyard (this really only works if you're outdoors ;) ) and the "Traffic light" (the person chosen to yell "Red light or green light") is all the way at the other end. The person who is the traffic light turns their back on the group of children and yells, "Green light!" The children run toward them, but be careful. If the traffic light turns around and yells "Red light!"all the children must freeze. Anyone who keeps moving or falls must go back to the starting line. The first person to reach the traffic light wins.
Hopefully you have an even number of children, or a very nice adult to play on someone's team. Split the kids up into either groups of two or just two large groups. Choose an object (like a baton) that must be passed from player to player. With team mates standing on opposite ends of the field they must run to the opposite side to hand the baton to their teammate who must run back to the opposite side. If you're using more than one teammate on each team the group whose players finish first win.
Three legged race:
I would only play this game only with older children. Set the kids up into groups of two. Have them stand next to each other & tie their legs that are next to each other together. Have the groups walk together to the starting line (this was they have an idea of how to start walking in the first place) The first group to get to the finish line wins.
You'll need either real sacks or pillow cases you do not care about for this game. Distribute a sack to each person playing at the starting line. The players must hop their way to the finish line. First person there wins.
This is more aimed toward older kids ( although my best friend and I did this to my husband for his 21st birthday when he knew what he was getting for his gift). First decide how many places you want the hunter to go. This will help you come up with the clever rhymes to send them on their search. Make sure you number the envelopes and write in small letters where it is supposed to go (not where they are supposed to go. Where you want to put the envelope).
Before the party place the envelopes in the correct spots & when it's time hand the first envelope to the kids.
An alternative to the scavenger hunt is just making a map of the backyard (or where the party is being held) have them walk 10 steps (you can mix it up by making them do Kangaroo hops to the swing set, or walk backwards 5 steps to the sofa)