To make your "garden", simply clean out the jars and then take a walk around your nearby park or even your backyard. The kids can place soil, sand, and cute rocks they find to make the bottom of their jar surface. At many of the dollar stores you can find small fake plants, decorations, and even some plastic animals to use to decorate their garden. Place the lid back on and you have a cute little decoration for their room. Pour it out and remake them when ever you need a quick activity.
These can also be themed for holidays to serve as decorations for the table. Place holiday colors and decorations inside instead of nature themed terrariums.