In my limited experience from about 8 years ago, and bearing in mind it was a birding trip so not exactly everyone's cup of tea...
Cancun is like Benidorm. Massive strip of hotels, junk food and crappy night clubs. Best avoided unless you like that kind of thing.
Cozumel is a short (about an hour?) ferry from somewhere south of Cancun. Regular and easy to do. Watch out for flying fish from the back of the boat! Cozumel itself was OK although I almost got thrown in jail (long story). Can't tell you about the diving/snorkeling personally but it's got a good reputation.
I went to Chichen Itza, Tulum and Coba but my favourite ruin site was Pelanque (a bit further south). But all the ruin sites are excellent places, not only for the Mayan culture but also for wildlife. The way all the ruins have been protected (same in most of Central America) means the areas around the ruins - generally primo lowland jungle - has also been protected and they have become real wildlife havens. Look out for some great birds but also howler monkeys, coati (aka: coatimundi), snakes, iguanas (fairly common, you should get cracking photos Matt), giant beetles, all sorts. Awesome!
Driving can be fun! Generally it's pretty safe and easy, but you do want to learn a bit before you go.
There are dire warnings about driving through rural areas after dark but we never had a problem.
A few things threw me somewhat - they tend to indicate in the opposite direction they are heading in sometimes, that sot of thing. But it's a piece of piss compared to some places (India, Thailand, etc).
If you do drive, look out for scams. This may not be as prevalent as it was in the past, and may be more common in the rural areas, but it pays to be aware of them.
The police are poorly paid and tend to prey on unsuspecting tourists. The classic is being pulled over and told you have just gone through a red light, despite the fact you might not have been through traffic lights all day. In general they want you to give them a couple of hundred dollars or they will "take you to jail". We got pulled over several times and came up with a novel way round it. The one Spanish speaker in our group would translate what the cop was asking for whilst the rest of us all talked loudly and excitedly in indecipherable Leicester accents whilst making wild hand gestures. We kept this up for several minutes until they just threw their hands in the air and told us to feck off. Worked on at least three occasions, that one.
Another one is at the petrol stations - count your money, out loud, into the attendant's hand, and do the same with the change you get back. We got stung the first time but soon learnt.
All in all it's a cracking part of the world with lots of fun things to do. You should have a blast! :icon_cool
