A date doesn't have to cost much or take all day. What she'll remember is that you planned it just for her and your phone stayed in your pocket. Here are ideas sorted by what you've got.
Free, at home
A pancake breakfast where she's the chef and you're the customer.
Build a blanket fort and watch one movie she picks, inside the fort.
A backyard campout with a flashlight and a list of stars to find.
Draw each other's portrait. The worse they are, the better.
Teach her something you know how to do, like changing a tire or your best grilled cheese.
Make a playlist together, then have a kitchen dance party.
Free or cheap, out of the house
Walk to get donuts and let her pick the route.
The library, where you each pick a book for the other one.
Watch the sunset somewhere with hot chocolate in a thermos.
A long bike ride with a snack stop halfway.
A thrift store challenge: $5 each to find the best strange thing.
Skip rocks, find the best stick and see who spots the most birds.
A bigger outing
Her pick of restaurant, and she orders for you.
Mini golf, bowling or a batting cage. The loser buys ice cream.
A matinee with the big popcorn.
A paint-your-own pottery studio.
A ballgame, a play or a concert she'd actually like.
An indoor climbing gym.
What works at each age
Under 6
Keep it short, around an hour, and close to home. The part she'll love most is having you all to herself.
6 to 10
Let her plan part of it. Give her a budget and a few options, and let her run the day.
Tweens and teens
Fewer questions and more side by side. Car rides, cooking, a show you only watch together. Let her bring the idea and you bring the time.
The hard part usually isn't the idea. It's getting it on the calendar before the week fills up. That's what Halftime is for: five minutes on Sunday to pick one real moment with each kid, one with your partner and an hour for yourself. It's free, there's no account, and it stays on your phone.