Pickup some of her very favorite donuts/ pastries, or some of her very favorite chocolates (in a small box just for her). At least $10 worth.
Plan 1 -- call the babysitter (if your kid needs a sitter), get pizza or take-out for the kid and the sitter, and take your wife out to a nice restaurant (preferably her favorite one). Dinner + drinks is good...
Plan 2 -- the backup plan. No babysitter, but most importantly no cooking for your wife. Take a drive and get pizzas or take-out (so everybody's happy), along with a nice dessert (try to get one of her favorite desserts).
Good luck!
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