Pack a cooler with fruit, cheese, bread, a bottle of your sweetheart's favorite wine, and drive to the shoreline. There is nothing more romantic than sharing a bottle of wine while gazing at the stars and listening to the ocean. If you feel like doing your research, plan it on the night of a meteor shower to make it even more memorable. Remember to bring a blanket; you'll want to cuddle when it gets cold.
Escape your everyday routines by driving away from them, at least for a few hours. Every couple deserves to see something beautiful together, so why not make a day of it? Our cars have evolved into errand machines at this point, so it's nice to use them for entertainment purposes once in a while. Make a road trip playlist featuring each of your favorite songs, but turn the volume down on the trip so you can hear each other. Be sure to take lots of pictures and stop at every lookout point.
Cater to your romantic partner's interests. You'll never understand why he wants to go to Comic-Con or how she can actually enjoy the opera, but while you are in no way obligated to like the same things, you are committed to making your romantic partner happy. Plan an evening full of activities you don't necessarily "get," but are willing to participate in. If you find yourself bored, look at how happy you made the love of your life and you'll enjoy yourself all the more.
Find places that have figured significantly into your relationship -- where you met or your favorite hiking trail -- and hide waterproof containers of memorabilia nearby. Introduce your romantic partner to the concept and give hints as to where you've hidden each container. If you want to up the tech factor, register each area as a geocache -- see the Resources section -- and use a GPS to find each item. Your memory and effort will be appreciated each time you find a container together.