cupsanddowns is a game where one team tries to turn all the cups upside down and the other team tries to turn them right side up. At the end of the time, whoever has the most their way wins. Simple, inexpensive game that could bring a lot of energy.
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!