First thing to decide on is what type of 'plinking', then choose a gun to suit. If just tin cans etc. then a CO2 powered blowback pistol will give plenty of fun and is easy to load - put 15 BBs or so into the magazine and shoot away. Lots of different models to choose from too.
For something more 'serious' then a CO2 target pistol can be used for competition type shooting. Again. easy to load and quite a few to choose from - budget to serious money PCPs.
I use a variety of pistol types, from SSP's through springers and enjoy them all and shoot monthly, at home, in the UBC competitions as well as enjoying fun plinking with friends.
My wife isn't interested at all in shooting but if she was and wanted a pistol I think I'd get her a Colt SAA. Not too heavy, easy to load and suitable for some of the UBC competitions.
Beat of luck - and be warned your wife may become addicted and there is no known cure.