rgbfoundry:
In addition to diving, I'm hoping to visit some big ruins. I know there are a few spots on the island, but do they pale in comparison to what's available on the mainland? If we go the mainland route, can you recommend some sites, or daytrip operations based in Cozumel?
The ruins on the island are nowhere near as elaborate as those on the mainland. You can go see them fairly easily, though, maybe as an adjunct to an around-the-island excursion (which I highly recommend).
On the mainland, the Big Three are Tulum, Coba, and Chichen-Itza.
Tulum is beautiful; it is on a cliff overlooking the water, and the area is cleared and mowed. The ruins themselves are fairly extensive, and it is located about an hour's drive south of Playa del Carmen.
Coba is far less excavated; you'll go through "tunnels" in the rain forest to get from building to building. Coba has the tallest pyramid on the Yucatan, and although only one face of it is cleared, you can climb it to get a view of the surrounding area from above the forest canopy. Coba is about 40 km inland from Tulum (about an hour on those roads).
If you ferry across to Playa del Carmen fairly early and rent a car, you can see both Tulum and Coba in one trip. There are tour buses, too, but you'll travel on their schedule, which will involve them placing you for large blocks of time where you can shop, and I don't know if any will take in both Tulum and Coba.
Chichen-Itza is in the interior, about 100 miles west of Cancun. It is the largest and most restored of the three, and pictures of the grand pyramid there are what you'll see come up first on just about any search for info on Mayan ruins. You can drive there from Playa del Carmen, too, but it's about a 4 hour drive each way. That's the way I saw it, but that's one where a tour bus might have been a better option.
All three of these sites are definitely worth seeing for anyone who is even a little bit interested in Mayan history. However you get to them, it is pretty much necessary that you hire a guide to explain what you are seeing. If you go on a tour bus, then it will be part of the package; if you rent a car and drive, you'll not have a problem finding one.
Enjoy your trip!
Cheers,