I have been meaning to try this for a year now. I took a card that I received this past Christmas and punched holes around it. Then I trimmed up the edge so that the holes were a certain distance from it, and so that the corners were rounded. Then about 2 feet into the yarn I did a blanket stitch around the card (through the punched holes). I started at the top center of the card. Using the side of the yarn attached to the ball (the opposite side of the yarn as my needle was on), I single crochet into the bars of the blanket stitch. Around the corners I single crochet into the same bar two times, chained one, and then single crochet twice into the next bar. I joined my round with a slip stitch, then chained one, turned, and crochet around it for a 2nd time. At the corners I single crochet into the corner's chain 2 times, chained 1, and single crochet into the same spot 2 more times. Then I slipped stitched to finished the round. Next I chained about 30 and then joined it with a slip stitch to form the loop (which is why I started the whole thing in the center top of the card in the first place).
This one is just one card, but you could hold 2 cards together (wrong sides together) when you make one. That way there would be a picture on each side. However with one card you could right a note on the back side and use it as a gift tag. Then the person receiving it can put it on their tree.