If you can cook, make a meal that's her favourite, bring a nice bottle of wine (I suggest a light chilled white unless she likes red or doesn't like wine), find somewhere outdoors that's nice and private to set up a sunset dinner picnic. Bring bug torches, stop for a fancy coffee drink to go to bring to the spot you have planned on the way there. Eat dinner, relax away from other people (bring pillows, put down a tarp, then a blanket in case the ground is cold/moist).
Does she like video games? Books? Jewellery? A massage is a good bet. Back, feet, etc.
If she's not into outdoor stuff, set up a little table in an open room at home (take out furniture temporarily to make it open if you have to) and do the same stuff basically, and go out for fancy coffee and dessert after if you can't make any at home on your own.
Does she live with you? After cleaning the whole bathroom really well, light some scented candles in it of a scent she likes. Run a hot bath and put a few drop of a relaxing essential oil scent she enjoys in it and have it ready for her when she gets home when you know she doesn't have to do anything else. Try to have dinner ready after this if you can.
This is just a pile of suggestions here. Basically, take time to do something special for her and take away any responsibilities and worries from her for a night. Don't just throw money at her in the form of going out for dinner or gifts, if you can help it. Gifts can be nice, but quality time for her and for the two you that you took the time to do yourself will mean a lot (if it doesn't she ain't appreciatin'!)
Though not everyone has the same ideas of romantic but if you think she'll like it, take her horseback riding for a day if it's available where you are, or to a vineyard for wine tasting and a tour, or on a fishing/canoe trip for the day, or a hike and pack all your food and bring some treats you know she likes. Take her to a band she likes, go see a museum or zoo of some kind, travel a distance with her to go shopping somewhere with a better selection or specialty stores she may rarely get to go to and patiently wait while she tries stuff on and hold her purse and help her find something that looks great without being a pill about the shopping trip.
Go to a big farmer's market and get some grocery shopping done while enjoying the food wares, buy a few specialty chocolates, go picking fruits/vegetables that are in season, go to a big bookstore and spend time browsing, then chatting over coffee. Take her "home and kitchen" shopping for a few things to brighten up a room around the house or for something in the kitchen that she's been dying to have around (ice cream maker, espresso maker, popcorn maker + movie night, waffle iron + breakfast, etc.)
I'm not sure if I have more ideas atm.
Sincerely,
A Woman