Surprise date night? As in take her to a restaurant, maybe romantic walk outside if the weather is nice or someplace else she isn't expecting. Maybe find someone to take the kid(s) overnight so you guys can fully enjoy the night. Give her a full body massage without expecting anything in return.
A variation on this idea, plan a surprise weekend getaway. Doesn't have to be expensive but maybe someplace you can drive to and just hang out and relax.
Is there anything she is really passionate about? I'm really passionate about cats and donating to shelters. For my wedding I asked guests to donate to a shelter rather than bring a gift. Not sure if anyone did. If she's into something like that, a donation in her name could be a good gift.
Does she like flowers? I know it sounds cliche but my favorite flower is a type that can't just be bought at any store. My husband special ordered them for me when he proposed and that meant a lot.