Give your child a specific task, like poking seeds into the soil or ripping weeds out of it. (Just make sure to let her know what not to pull out, like your prized marigolds.)
One word: mud. Add a little water to a patch in the backyard (or to a bucket filled with dirt) and let her squish her fingers in it, paint it all over her arms, or make sculptures with it.
Show her how to dig holes. It'll keep her occupied for ages.
Switch on the hose and start spraying. She'll be giggling in no time.