Check out Science for Citizens to find an opportunity that's right for your family. Some of my favorites:
Mastodon Matrix Project
It doesn't get any cooler than this. They ship you a sample of mastodon fossil matrix (the stuff the fossil is found in) and you sort through it to find ancient bones, plants, and rocks from the time the mastodon lived. This would be a perfect project for a science club.
Maybe your kid isn't a science lover but rather a budding musician? Encourage an interest in science by suggesting he write a song about a physics concept to add to this database devoted to collecting and organizing all songs about physics.
I could spend hours (in fact, I have!) watching the squirrels in my back yard chase each other up and down our trees and in and out of our compost heap. I was intrigued one afternoon to watch a squirrel trying to bury a corncob in our lawn. With Project Squirrel you can report your squirrel sightings to help scientists learn more about the charming neighborhood animals.
Monarch Waystation Program
According to the program's web site,
Widespread adoption of herbicide-resistant corn and soybeans has resulted in the loss of more than 80 million acres of monarch habitat in recent years. The planting of these crops genetically modified to resist the non-selective systemic herbicide glyphosate (Roundup®) allows growers to spray fields with this herbicide instead of tilling to control weeds. Milkweeds survive tilling but not the repeated use of glyphosate. This habitat loss is significant since these croplands represent more than 30% of the summer breeding area for monarchs.Help save the magnificent Monarch Butterfly by creating a Monarch Waystation in your backyard! Find out how at the program's web site (linked above).