Ruger 22/45 w/4" bull barrel or Buckmark
I've got a KMK-678GC and it is a great gun, but looking back, for plinking around, I'd get a bull barrelled 4" model instead. The Mark II is more solid, but after going back and forth between my CZ and Mk II a few times, I'm starting to think that the 22/45 would have been better because of the similarity in controls and grip angle - but I'd have to do something about that crappy polymer grip. Some skateboard tape or AGrip would probably be enough.
The Browning Buckmark is a nice little gun as well, and the buckmark micro looks really sweet. Plus, the grip angle is more familiar. However, some folks claim that the Buckmark is not as solidly built as the Rugers - but the Buckmark's trigger is supposed to be nicer. When I shot the Buckmark and the 22/45 side by side, I much preferred the Buckmark - mostly because the Buckmark had much better feeling grips.
If you want to do some competitive target shooting, the 5.5" barrel versions of both are supposed to be more than adequate, especially if you put a dot sight on them.
If you go with a Ruger, pick up a Volquartsen target sear ($18), an OnTarget trigger shield ($7) and polish the feedramp as well ($1 for sandpaper) - then you're all set.
The Buckmark, out of the box, is probably going to be more accurate, familiar to shoot, and better looking as well - but maybe not as durable. The Rugers requires a little more DIY work to compete with the Buckmark but with reasonable care, you could be teaching your grandkids to shoot with the same gun.