I bought a strobe target indicator that attaches to the back of steel targets for the Pawnee Run 'n Gun, and found it very satisfying to see that strobe pop when you whack steel at 500 yards!
Spuds has to be the cheapest of the "natural targets". I was at the store today, and a 5lb bag was $2.99. The 10 lb bag on sale was $2.99. My old bag is getting dried out, so they go to the shooting berm in the back yard. In season, tomato's are pretty fun.