Have you ever done iris folding? My sister-in-law, Susan, taught me how to make these cards a few years ago. I use the heart pattern for wedding cards, but it would be ideal for Valentine's Day too.
You can download the free heart pattern here. The instructions are no longer available on the Circle of Crafters site, but there are several online tutorials if you do a search.
This pattern fits perfectly on a 5" square card and then in a 5 1/2" square envelope.
Here are the basics:
First, print out two of the patterns--one to cut up and one to use as a reference.
You will also need three different papers cut into approximately 1 1/2" strips.
Trim the overhanging edges. Turn it over and be amazed. Use double-sided tape to attach the finished product to the white card.
I made four different cards today. I like the black and white ones best.