What about writing him a nice poem? Or leave him little "love" notes all over the house for him to find. Play a special song, or you can even make "healthy" apple pie as a quick little dessert! All you do is slice up your favorite type of apple, stick it in a mug, and pour a little cinnamon, vanilla, and brown sugar (You can use the 50/50 brown sugar splenda mix if you're on a diet!) and stick it in the microwave for a few minutes until the apples are soft and juicy. Then simply enjoy!
For me, the little things are more special than anything big or extravagant. Is there a special movie he might want? You could buy it for him and watch it that night. Sort of off topic, but it meant a lot to my dad when I watched his favorite movie with him, even though I didn't want to!