1) Make each other cards with crayons. You can do this as an activity together or your cards can be pre-made.
2) Go on a walk and pick your Valentine a flower. Make sure and present it to him/her with over-exaggeration.
3) Watch a cheesy valentine movie together. The more goofy romantic the better.
4) Make each other the "coupons" with funny things like "I will pick up your socks one time without rolling my eyes".
5) Cook spaghetti for dinner and try to do the lady and the tramp maneuver with a piece of spaghetti.