DIY - Glow in the dark pots with mums for fall/Halloween. Buy a flower pot that you really like and use Rustoleum's Glow-in-the-dark paint to paint the pot. During the day, the paint will absorb the sunlight and at night the pots will glow.
Make a backyard bowling alley. Add food coloring to clear plastic bottles of water. Use party streamers as lane margins. Add glow sticks for night bowling. (These would make great outdoor table nightlights also with the glowsticks, gonna try it!
Outdoor shower/water day for kids.Could easily be made with hollow box uprights, run the water tube through one and the top hollow box with a PVC tube that you drill holes in for the water to run out. Faux paint it to look like copper, really pretty easy.