This is what the website says about what to do with the playing cards.
The playing cards
If you have come upon this site as a result of geocaching or finding a geocache with one of our marked playing cards, here's some information about the cards:
Basically we wanted some way of keeping track of the people who visited the caches we did as well as getting people to read the blog, so what we ask is that if you find or have found a card that you contact us by posting a comment or sending us an email on merrydowners@gmail.com and tell us where you found it and a little about your adventure getting there. Then we would like you to deposit it in the next cache you find and allow it to travel some more. We want to see how far we can get them!!
