+1 to the .22LR w/ potatoes! I figured this out in college when I had a bag of potatoes that had started to go bad and I wanted to go shooting. I grabbed the potatoes, some pop cans, and various other targets. I couldn't believe how well the little federal bulk pack hollowpoints popped those potatoes. It's loads of fun for pennies on the dollar.
It's also, in my opinion, one of the single best ways to introduce new shooters to the sport. It's cheap (especially when potatoes are in season as you can get a few pounds for $1 around here), the targets are reactive, there is no need to clean them up (unless you want to) and there is no loud noise or recoil to intimidate the newbies.
.22LR can get boring, but if you don't shoot it too often it's fun to "come back" to it.
Otherwise, I'd say 7.62x39 is my fun plinking round. If I had a .223 it would probably be a dead heat between the two.