Another suggestion if you're short on money: Make it something themed and elaborate where the planning is the greatest gift!
Example: Hide a folding table and two folding chairs in a beautiful, romantic place out in nature. Have a clean, white tablecloth, a chilled mini bottle of sparkling wine (=cheap), two champagne glasses, a bottle of chilled water and two water glasses, and a yellow-frosted mini birthday cake with a candle on it (=$10) and matches hidden in the trunk of your car.
Pick her up and blindfold her, then drive her to the romantic spot. While she's still blindfolded set up the table, chairs, tablecloth, cake and champagne, then seat her blindfolded and open the blindfold when the candle is burning!
Then whip out a small gift ($20) with a romantic inscription. E.g., buy a small gift set with 3 mini sunscreens from a good brand (e.g., Clinique; should be around $20-25), and add an inscription "A little sunshine for my sunshine girl!"). Give it to her, pour her the "champagne", serve the sunshine-colored cake and kiss her -- voila! It'll probably be her best birthday ever.
You can hardly go too over-the-top with women, even the ones who'd never admit to it!
Good luck!!