A fun scavenger hunt requires little in the way of money or resources. All you need is some advance planning. Print up a list of things for your kids and their friends to find: these can be people, places, or things. Then, set them loose (with parental supervision, if necessary) to find their targets. The winning team gets a prize.
A variation on this is the treasure hunt, in which you plant treasures for your children to find. Each one has a clue that leads them to the next one. Most of the treasures can be small, like candy or small toys, and the final treasure can be a real treat, like a new video game or dinner out.
Read the instructions for a family scavenger hunt.