Kids especially like reactive targets. Things I've used:
Shaken cans of the cheapest soda I can find (Sam's club, etc.).
12 oz water bottles refilled with tap water and caps screwed on tight.
Ritz crackers, set up in a grooved length of 2x4.
Balloons.
Ice cubes.
Spent shotgun shells.
Tin cans upside down over a stake (use a wood stake, not steel); can rattles nicely, and you don't have to go set them up again.
Clay birds set at various distances.
As others have said, pick up your trash.