If you just want to plink at cans in the backyard, get a BB gun. No license required and way cheaper than an air rifle here (.22 air rifle will cost about the same, if not more than, a .22LR rifle).
And unless you live close to a shooting range, no point in getting a 'real' firearm licensed to your wife because you'll not be shooting in your backyard. Even if you do live near a shooting range, officially the registered owner of the weapon needs to be present (although it's very rare for ranges to check that someone who brings a gun to shoot is actually the registered owner).