I always appreciate holidays because they give an excuse to be creative and come up with a fun way to celebrate--especially on a grad school budget. Since the stereotypical beautiful bouquets of flowers, nice dinners, and expensive colognes aren't in the picture at the moment, we tend to have an exciting array of crazy ways we celebrate.
For Valentine's Day, I wrote a poem and cut it apart into 25 sections. Then I folded each section up and stuck it in a balloon, along with a mini candy bar. I blew up all the balloons, and the night before I put them in the car for Sam to discover as he left for work.
He had fun popping balloons at red lights on the way, and he worked for over an hour and a half unscrambling the poem when he got home!
He had to work late, so we enjoyed one of his favorite dinners late-night style while he worked. (Recipes here and here, if you're interested.)
Sam got pretty creative, as well--you can tell your husband is a scientist when he leaves you a secret-message Valentine written in baking soda and e-mails you instructions to spray it with grape juice to cause a chemical reaction to occur and turn the message green!