Some ideas I just thought up now...
-Cook a healthy meal together, then for dessert take a joy ride to a scenic spot and make out in the back seat like teenagers
-Make a photo album or scrapbook of your favorite memories and inside jokes and give it to him
-If you sing or play an instrument, learn his favorite song and play it for him
-Write him a poem or short story, make a really detailed card chock full of memories/inside jokes/feelings
-(Not diet friendly) Order in and just veg at home, you can do a lot of fun things at home that don't require spending money

-(Also not diet friendly) Go to the most expensive restaurant in town - for dessert only
As for my own plans, I'm not sure. The weekend before Valentine's Day I'm going to Korea with my friends so I won't see my bf. In Japan, for V-day girls give chocolate and presents to their boyfriends/husbands/male coworkers, and one month later (called "White Day") the men reciprocate. I like this system so we have been celebrating Japanese-style for the past 3 years while living here. We're both dieting so making a pan of brownies and eating until we're sick like years before is a bad idea.