1) Grab a few blankets and a thermos of hot cocoa and head to a park. Huddle up under the blankets together and watch the snow fall while sipping hot cocoa.
2) Take a romantic walk in the rain. (Note: I would wait until summer to do this.) Don't forget to take an umbrella, though.
3) Surprise her as she's coming out of work with a bouquet of wild flowers that you picked yourself and a pizza. Go on an impromtu picnic. If you don't want to pick the wild flowers, I recommend a bouquet of daisies.
4) Take her to a pet store to look at the puppies and kittens. Note: this may backfire, however, and you may be coming home with a yapping machine. However, this one was one of my favorites back in high school.
5) Rent all a few of her favorite movies, go buy a big tub of movie theatre popcorn, and surprise her with a movie night.
6) Take a drive to somewhere secluded, park, roll down your windows, turn the music up and ask her to dance with you in the moonlight.
7) Go for a moonlight walk on the beach. I recommend bringing that thermos of cocoa and maybe finding some firewood on your way. Cuddle up next to the fire and watch the waves crash on the beach.
8) Surprise her at home by cooking her favorite dinner. Afterwards, do all the cleanup for her, too.
9) Borrow a book of romantic poetry from the library, go someplace quiet and read to her.
10) Channel your inner youth! Have fun in the snow! Build a snow man together, have a snowball fight, go sledding! (Again, that thermos would be pretty handy)
11) Back to the beach. Build sandcastles together.
12) Take photos of the two of you and spend the afternoon making an album out of them and reminiscing.
13) Go do kareoke with her. Sing her favorite song to her. She'll think it's romantic and silly no matter how horrible you sound, regardless if it's a Spice Girls tune or not.