I purchased the thinnest clear vinyl I could find at Wal-mart. At $.96 cents a yard it was quite inexpensive. I thought that this would be a great lacing project for Cyber Princess, but our hole punch couldn't cut through the vinyl, so I ended up using a darning needle to sew the ribbon through.
Since I had to sew through the vinyl anyways, I used leftover fabric for the Valentine backs. Next time I will use page protectors because they are easy to hole punch (and paper for the back)!
We attached a tag with the ribbon to label the to and from for each Valentine. While the satin ribbon I used was easy to sew with, it didn't stay tied very well. In fact I had to reattach most of the tags at the party, so that we could "deliver" our Peekaboo Treat Valentines. I would avoid using satin ribbon, for this project, unless you know some secret to keeping the bows tied, but still untie-able.
Rating: Cyber Princess gives Peekaboo Treats 5 fingers. She was super proud of her Valentine's. Since I couldn't hole punch the vinyl and basically had to sew the Valentine's without much involvement from Cyber Princess, I would only give this project 3 fingers.
However, I was able to read in the comments section of the project on FamilyFun's website and learn that many people had success using clear vinyl page protectors. I am sure that if I had used page protectors, we could have worked together to lace the cards and I would give the project a higher rating.