Costa Maya was one of the ports on a cruise I went on 2 years ago and from what I remember there wasn't much to see or do there. It basically consisted of a small shopping area and a beach (truly, I'm not exaggerating). If your looking to explore interesting tourist attractions I think you'll be disappointed. Unless things have changed in the past 2 years, I suspect renting a scooter will be a waste of your time and money.
As for Cozumel, I haven't done the Dune Buggy Tour so I can't give you an opinion on that.
We were there in April 2005. At the end of the pier is the shopping malls, pool with bar, restuarants and there is some entertainment at various times in the square. We rented a golf cart ($15/hr) and went to Mahahual which is about 2 miles away and hung around there for about two hours. They had a very nice beach there, and again, more vendors. I found the vendors there to be pushy. Had my arm grabbed a couple of times by vendors trying to pull me into their stalls which I didn't like. We're going back in April 2005 and have decided to take one of the tours the ship offers. If you contact Carnival they will send you a list of the tours. Hope this helps.
Celebrity Century 1/09
Carnival Destiny 10/07
Carnival Destiny 9/06
Monarch of the Seas 3/06
Carnival Inspiration 8/05
Carnival Miracle 4/05
Carnival Inspiration 4/04
Sovereign of the Seas 4/03
Dani - I get a little over agressive when it comes to vacation planning - I know it's a bit early to have our excursion booked - but hey why not! Last year we spent a week at an all-inclusive in Jamaica and I had a personal tour guide booked for a trip off the resort by May, we didnt get to Jamaica until late December!
Darcy - Thanks, Carnival doesnt list anything - I will contact them and see what they have to offer. I'd be happy though with just a relaxing day at the beach!