If you want to do ruins without a lot of walking and climbing, Tulum might be right for you. We booked through Carnival so we wouldn't have to worry about making it back to the ship. When we arrived in Cozumel, we boarded a ferry directly from the ship to the mainland, then a bus to Tulum. There is even a tram car ($1 pp) that will take you from the parking lot to the ruins. There are shopping and bathrooms in the parking lot and the bus makes one stop each way for the bathroom and souvenier shopping. My mother is 69 and did okay except the heat at the ruins got to her a bit. A hat would have helped. Also, the (GORGEOUS) beach is accessible after the tour but only by a long set of steps.
If you want a beach day, on another cruise to Cozumel my friends and I took a taxi (walk through the port and you will easily find the taxi stand) to Paradise Beach. The beach is free as are the lounge chairs and umbrellas. There is a small fee ($5 I believe) to use the floating iceberg, floating trampoline, and snorkeling equipment. There are banana boats and other items for rent as well. There is an open-air restaurant you can sit in but the waiters will bring you food and drink directly to your beach chair. We had a phenomenal time and my friend went back with her hubby and teen daughter and they loved it as well. There is also shopping and we did not get harassed to buy while we were on the beach or in the shop. There are changing rooms and outdoor showers. Here is a link:
http://www.paradisecozumel.com/
I have been to GC twice and both times booked a trip for snorkeling and stingray city through Soto tours. It's a once in a lifetime experience. They take you to one stop for snorkeling and then on to stingray city where you feed, touch, take pics, etc. with the stingrays. My mother has never snorkeled before and had to get back onto the boat as the waves were too much for her but stingray city is only knee/waste deep and she had the time of her life there. After you tender in off the ship, you walk less than a block to a parking lot where a van picks you up and takes you to the boat. They provide all the snorkeling gear, water, fruit punch, bathroom facilities on the boat, and will rinse you off after the tour. They then take you right back to the ship and there are several stores/restaurants right in that area if you want to explore a bit without lots of walking. Here is a link for Sotos:
http://www.sotoscruises.com/about_us.html
You book online with a CC but then pay cash or traveler's check when you arrive. Unless of course things have changed.
SOMETHING VERY IMPORTANT TO BEAR IN MIND: GC and other places do not observe daylight savings time. If you book a trip through the ship, this is not a problem. If you book on your own though, you must check and see what time it is in port versus the ship. I know that when I was in GC in May, they were either an hour ahead or behind the ship and so our reservations for Sotos were an hour off the ship time. Not a problem as long as you are aware.
Hope some of this helps and have a fabulous time!
