Sorry in my post I said 45 minutes but we were driving a rental car so we probally were going much faster than a tour bus, anyhow it's a good haul!
If you are athletic we did an Eco tour on our last trip to Cozumel in June. IIt does involve ferrying over to Chick Ha on the Yucatan but still leaves some time to enjoy the island later in the day. We jungle trekked on bikes, zip lined, snorkled and tubed in natural cenotes, enjoyed a Mayan lunch and purification ceremony in the village and were back in Cozumel by 2:30 in the afternoon. Plenty of time for us to hit a small place for a Margarita and persue the shops and purchase a sink! To this date is one of our most memorable excursions and we've cruised quite a bit.
Whatever you do have fun.