Maybe it's just me, but I find shotguns really fun. Maybe buy an old 870 Wingmaster or even a Winchester 1300? Heck, since it's just for fun, tacticool the heck out of it.
Kel Tec SUB2000. Most likely you can find a model that shares magazines with one of your handguns. Folding mechanism is cool and ammo is relatively cheap(in 9mm at least).
Or call me crazy, but what about one of those Braztech/Rossi matched pair single shot .22/.410 combos? Yeah I know it's not American but hear me out. After all kinds of more serious firearms, I wouldn't have thought I could have fun with a single shot .22, but I shot my 6yo nephew's and had more fun at the range that day than with anything else. Getting back to basics is satisfying and I love how it ejects the spent brass right over your shoulder when you break it open. Cool. They also have them in many different caliber/gauge combos. Cheap too at about ~$175 if I recall.