I first saw this craft at the Artful Parent blog. Take white coffee filters and cut them into a heart shape and then use water colors or do-a-dot markers to completely cover the filter with color. Once they dry (about 10-15 minutes) they look beautiful hung in a window where the light will shine through.
Don't work about the crinkles in the coffee filter paper they completely disappear once they get wet with paint. Such a simple, cheap and beautiful craft.
Step 1 - cut coffee filter into a heart shape.
Step 2 - watercolor the filter paper
Step 3 - let it dry and then hang in a window
This craft along with other heart crafts are featured here Hearts Arts & Crafts!