Terrariums seem to be the rage these days. They're actually quite easy to put together, requiring only a few simple materials.
I hosted a terrarium party a while back for moms and kids. Moms made larger terrariums and kids made tiny, jarred terrariums. We placed miniature princess figurines in our terrariums, but you could use any small characters.
The kids had so much fun placing and arranging the soil and plants into their jars. If you take good care of your plants, they can last quite a while- ours are still growing after several months!