Long walk together (scenic or even in your own neighborhood!)
Run errands together but without kids (women love to finish their errands and have company at the same time!)
Go out for coffee and dessert instead of dinner
Hit a hiking trail

Go wine tasting
Take a dancing class
Sign up for a one-night class at your local community college for something she wants to do/learn
Pack a picnic and find somewhere scenic to enjoy (the key here is that you make the preparations!)
Take a dinner cruise

Sing karaoke together!
Try out local theatre
Visit a museum (many have one free night a week; Seattle Art Museum has a great restaurant, Taste, for dessert!)
Shopping (this might not be your idea of a night of thrills, but for most women it is an event they really enjoy. You could make it worth your while and spend some $$ at a lingerie store!)