For our Service activity day, we did this simple, easy clean-up.

We met at a local park, and brought plastic shopping bags and gave one to each of the children.
Each teacher was responsible for 4 or 5 children, and we covered the park picking up garbage as we went.

The children were very enthusiastic.

Afterwards, we let them play on the playground equipment while we got an easy lunch ready: hot dogs, chips, punch, popsicles.
Be sure to have somewhere that the children can wash up before eating.

Relay games could also be played in the park, if desired. It was a very successful activity, and the children seemed to enjoy all of it.