Here's some of the sites we've used for ideas:
http://www.family-reunion-success.com/outdoor-lawn-games....
http://spoonful.com/summer/summer-games - if it's hot
http://traveltips.usatoday.com/funny-outdoor-game-ideas-f...
http://suite101.com/article/outside-family-reunion-games-...
But in all honesty, the kids are often difficult to engage in playing games, they want to do their own thing other than the pinata, it's the adults that get into playing the games! (And we do this every year.) They will do crafts at the craft tables, have their face painted and play with another person, so last year we set up board games and Legos, plus had several footballs, basketballs, and soccer balls for them to use in addition to the crafts, pinata and face painting.
We get things on clearance or on sale for prizes...bubbles, sidewalk chalk, crayons, jump ropes, balls, Hot Wheels or Matchbox cars, small bags of Legos, frisbees, water bottles, Slinky's, etc. I go to the Dollar Spot at Target and check their clearanced regular stuff, and craft stores like Michaels and Joann's, they have fun stuff all the time near the checkouts. And I always check out Dollar Tree and often find lots of fun stuff there.