Over-ripe watermelons! Other over-ripe fruits, out-of-date eggs. If you're in a "wilderness" area, no clean-up necessary, and the crawly critters will love you!
Helium-filled balloons for shotgun. Put a small amount of powder (talcum or ?) in the balloon before you inflate for a smoke effect.
Movie posters and/or full-sized printed cardboard cut-outs. Sometimes, the movie theatres will GIVE you their old posters or the cut-outs of actors. I got several full-sized "Crocodile Dundee" cut-outs with Dundee holding a Foster's beer, from a local liquor store. (Funny story: Before shooting Dundee, two of my buddies stood beside the cut-out for a photo. It looked like Dundee was a REAL person in the photo, and someone who wasn't there at the time asked, "How did you get him to go out shooting with you?")
Cardboard boxes or paper bags filled with shredded paper. Blast them enough, and it turns to confetti.
Mild steel plate suspended between trees. I have one that I've used for 10+ years, since it's only for .22's. "GONG!" Just bang out the dents and give it a fresh coat of paint after it has been "splattered" sufficiently.
Cheap 4x4 R/C vehicle. No, not the vehicle! Tie a long piece of string on the rear, then add an empty soda can on the other end. Moving target! When the soda can is full of holes, drive the vehicle back to you for a fresh target. This can also be used with helium-filled balloons that are tethered to the 4x4 vehicle. Just be sure to not hit the tow vehicle!
Rope and pulley: You need a high side and a low side. Attach a fairly long (3') piece of wood onto the pulley, then attach either wood or cardboard targets to the other end of the wood.
On the high end, put an "eye" screw in, thread some string through it, then tie it to the pulley. The string should be long enough to allow the moving target to reach the low end AND to where your shooting position is. Pull the string so that the pulley/target goes to the high end, then let go of the string and allow gravity to send it to the low end. Pull the string to re-set the pulley/target at the high end again. Great for handgun shooting at moving targets, but be sure to have plenty of "safe" shooting space!