For my daughter's last Birthday, she wanted to have a treasure hunt around our neighborhood.
I was able to talk some of my neighbors into giving out the clues as the kids came to their doors... I was even allowed to bury the treasure in the sand box of one of my awesome neighbors. It was a fun party and the kids had a blast running around the neighborhood in their different groups.
I made my own invite in photoshop and sent it to Costco to be printed, but the original idea came from zazzle.com. They have tons of options that you can add your own text to. I usually make my own stuff, if I can, since I have graphic design degree and need to use it for something, but I get that not everyone has that ability or option!