Sticky notes with sweet messages :) put them in odd places.. on her stearing wheel, bathroom mirror. find out her favorite flower and randomly give her some. Find a song that says how you feel or reminds you of her and tell her to listen to it. Those are a just some things I would like lol oo if you can, a picnic in the park, that is so much fun ;)
Try to put her favorite candies in her pocket without her noticing (works better when it's not chocolate! Jolly Ranchers or Starbursts are a good way to go). Maybe make her a mix cd with her favorite songs, that's always cute.