For sheer fun and entertainment value, I'd suggest the M1 carbine. I shot one that belonged to a friend a few years ago and it wax bar far the most fun I've ever had. Lightweight and yet Low recoil, ammo plentiful and not very expensive (at least it wasn't at that time), and powerful enough to be impressive on target.
Remember that you choice of targets will have a big effect on how much enjoyment you get out of your guns. It's ALWAYS more fun to shoot an object than it is to punch holes in paper.