A few things I've had tucked in my "Valentine's" idea folder:
This one is my favorite--simply beautiful and perfect--from Urban Comfort.
Another favorite from Urban Comfort.
Of course I love the polka dots on this Valentine from Paper Source.
I just added this delightful idea from One Charming Party. It's made from gumballs. My little girlies would love this. You can download a free set of valentines (including the one shown) on their site. This website is amazing. Check it out if you haven't already. The photography (by Nicole Hill Gerulat) and party ideas are amazing.
I found this one last night. A delightful and beautiful freebie from Rebecca Cooper on her blog Simple as That. I love the color combination, the fonts and Rebecca's blog. She also has a download for matching Valentine's notecards and a print. It's all right here.