I know there is a fabric store in Playa del Carmen. But that is a ferry ride from Cozumel. I too would rather have a piece of fabric souvenir than a t-shirt (altho Cozumel has some great jewellery shops
) You could also take a tour or even a taxi ride round the wild (ocean) side of the island--totally different and beautiful There are several places to stop for a meal or drink and just enjoy the quiet. But be aware that the those beaches are swim at your own risk.
As for the ruins, from Cozumel, you would have to take a ferry to the mainland and then it's a bus ride--probably an hour or so to Tulum or Coba and more like 2 to Chichen Itza. Definitely a whole day trip sort of thing. Oops, just reread your post. If you are stopping at Costa Maya, I think that is just south of Playa del Carmen so you would already be on the mainland. Hmmmm... interesting that you would stop at the mainland port and then go just a few miles up the coast to the Cozumel port.