Take a look at the Kel-tec SU 16 series. A SU 16A is on my list. It is a little cheaper than the mini, reported to be more accurate, takes AR15 magazines, folds in half to fit behind the seat and holds up well. Some might bash Kel-tec, but I've heard many good things about this gun. It fits your definition of fun plinker, cheap ammo and not too expensive.

My $.02.

Spud