Balloons are fun. They pop when you hit them, no worries. Get different sizes for different distances/level of challenge.
The small "tactical", I mean "personal" watermelons, they turn inside out when hit by .223
Cheap glossy wall posters from wal-mart, shoot Johnny Depp as many times as you want.
Golf balls
Hang tin cans from a string
At night, a small propane cylinder hit by a pistol caliber tracer bullet.
Half stick of dynamite. Works best with old appliances.
and, last but actually first,
Beer cans.