You could always explain that you're broke but still want to show your love by making a Valentines IOU, such as this one:
I'm outta money, and I have no gift for my boyfriend this Valentine's day!! What should I do? Sing him a song? Make him cookies??
Cookies is a good idea! Sexy time too. You can cook him a nice dinner. You can make something for him like cute cards or a drawing etc.