Doing some research, I've found several really fun ideas. I'm not sure which one I'll pick yet, but I think these are all super creative and possibly doable for me :)
1. Quilled Open Heart Valentines
This looks like the most challenging of the designs, but I love the result! I have no idea who sells quilling paper, though. (Michaels? Hobby Lobby?)
2. Queen and King of Hearts Valentines
Simple but cute. I could pair it with a homemade envelope for a little more cuteness.
Lots of complicated folding, but I think P would really enjoy opening this :)
I think I did something similar to this in Girl Scouts back in the day :)
Real Simple had several great Valentine's Day craft ideas. Love the button card! I need to buy some buttons anyway so I could just buy several pack of red ones while I'm at it.
The Sweetest Occasion:
This is my favorite, but I think I'm going to save this idea for our anniversary in April because I just don't have the time to do this right now! This project is called the "Library of Love" which, of course, is oh-so-fitting for me since I'm a librarian :) Basically you create library catalog cards replacing the call numbers with important dates in your relationship, book titles with memories and significant quotes, and publisher info with cities where you have traveled and then file them in a decorated box. I especially like this idea for an anniversary because you can add cards to it every year.
I'm also going to attempt to make cake balls for him as a little Valentine's Day treat. I tried to make cake balls for the first time for my office Christmas party, and they were a gigantic fail and I couldn't take them. But Peter thought the ugly results were quite tasty so I vowed to try again. I will not be defeated by cake mix, frosting, and melted chocolate :)
(images from Martha Stewart, Real Simple, The Sweetest Occasion blog, and Bakerella)