A really good scavenger hunt can keep kids entertained for quite a while. Buy some inexpensive prizes (enough for all party guests) to hide around the yard and take some time to come up with fun questions that aren't too difficult to answer. Lead the party guests through your living room or backyard by dropping clues along the way, and watch the fun ensue. Maybe your scavenger hunt leads them to a pinata? Maybe they find themselves ending up at the birthday cake? Be creative and make sure to estimate how long the hunt will take. Give magnifying glasses as party favors along with fun disguises like fake mustaches or funny noses. Make a game of "Guess Who?" where kids have to guess famous celebrities or TV characters based on a set of clues about each person.
Everybody loves a cool cowboy! Find fun western-themed invitations or design your own. Look for checkered tablecloths to eat on outside, put silverware in metal buckets, and decorate with hay bales. Give party guests bandannas, cowboy hats, or sheriff badges as party favors. Plan some exciting games like a broomstick horse race, a skilled game of horseshoes, or old-fashioned tug-of-war. Play classic country music in the background and serve barbecue chicken, baked beans, and sasparilla to complete the theme. If you really want to go all-out, consider hiring a truck to take your party guests on a hayride around the neighborhood (this might not work depending on where you live, and you'll probably need to get parental consent first). For an evening party, set up a backyard fire it and toast marshmallows to make s'mores.