HI T.:
A few weeks ago, we helped host a "pool party" at our neighborhood HOA that got rained-out, but one of the biggest hits were MINNOW RACES! Sounds funny, but, we had 2 rain gutters, and even put in a start/finish line. The kids got to fish out their own championship minnow and race them against one another. It was a lot of fun, and the kids kept coming back for more!
For my son's party, we held a party outdoors at Grandmas. We put out a variety of activities: t-ball, obstacle course, soccer, sand castles, and more. We asked the kids to roam from one activity to the next ... you could even do prizes? This could be organized, inexpensive, and fun for all ages. ~ Good Luck!