Most Helpful Girls
I don't know if guys like this and you'd need the right vehicle for it, but honestly drive out to a location where you can see the sunset. Have blankets and pillows in the back of your car and string some fairy lights around and cuddle with her and talk, have some food and if you decide to get her a present give it to her then. You can watch the sunset and it's romantic af. Bring some movies if you wanna. If I had a boyfriend and he did that for me I'd fall for him all over again. That's how much I like it. And if she loves the outdoors, even better.
Cook for her and buy a birthday cake with some beverages that you both like to drink, and serve the cake after she's done with the lunch/dinner. That'll be if she wants to be completely just you and her.
Otherwise eating out somewhere would be nice as well, and definitely birthday cake at the end of the meal, and if you have a material gift for her, give it to her then, or in the beginning when you meet for the day. I think these are pretty old-fashioned, but I would like these done to me by my boyfriend.
Or a movie followed by the meal, but make sure you time right otherwise you'll both be too hungry before the movie ends.
If your girlfriend is simple, she will appreciate these; if she wants fancy celebration then she's going to think that these are too cliched. And in the case if she needs something fancy, you could take her on a cruise dinner if you can afford it 😄