Plinking

[RobG]

Don't use glass bottles. They shatter and leave shards everywhere that always seem to get left behind.

[CSACANNONEER]

Charcoal, necco candy, grapes and other small fruits, old propane tanks (at a good distance), strike anywhere matches (the idea is to light the match with the bullet), cutting playing cards in half (already mentioned but, still a classic), coin sized pieces of metal, etc. These are all (except for the tank) things that have been used in shooting competions for generations.

[prob]

Ditto on the potatoes. Go tho the 99 cent store and get bags of potatoes or onions. They both react with a very satisfying splatter when hit. You don't have to clean the mess up either; they very quickly decompose. Bowling pins are surprisingly boring to shoot at. They just tip over and you have to go set 'em up again. Gets dull after a while

[sprintcar]

Bowling pins knock them down and keep them moving.
charcoal makes a nice little black puff. Cheap diet soda in cans = nice explosion but no sticky mess.

[KimoBBZ]

bio D grade

Old fruit is great (apples & melons especially) and cheap/free... hit your local grocery, they normally have stuff they can't sell. I've been using a lot of crackers lately... Ritz, saltines, soda, check a 99 cent store. Also, hard candy from Halloweens/Christmas past, a little peanut butter and you can stand up anything from a candy cane, to a jolly rancher.

No cleanup, just let the critters have at it.