If you are in to ruins then the trip to Checien Itza is worth it, although it is a long day. Bus ride will be about 3 hours each way from whre you are staying.
Coba is closer, and supposed to be an even larger pyramid. Only discovered about a decade or two ago and only opened to public in recent years. As Coba is not as developed as other sites they also have other stuff as part of the tour including zip lines. The Checian Itza tours give you more historical information. Every few years now they have been making more progress in decyphering some information, and there is not only more info now, but some of what they thought has since been changed by the new info. Quit interesting.
Xcaret is a great day if you are with SO and/or friends and/or kids. Don't bother with the tour to Xcaret. Just take a cab and buy ticket either there or before you go through your travel agent at the hotel. Stay for the night show - it isdefinately worth it. Also recommend you get the dinner package for the night show. Do the underground river and cenotes first thing, especially if you can get in before a the bus loads show up. Very cool, and the water is quite cool too. Tons of other stuff in Xcaret I won't bore you with here. They do have a website where you can even buy tickets in advance before you leave Canada and ata small discount.
The ferry from PDC to Cozumel is usually fun, but Cozumel sustained so much damage from the huricanes, they are still not totally back to what it was. If you do go to Cozumel, neat little 'Bavarian' bar near the square and beer only $1 US. Great place to take a little break. In PDC there are a couple ofrestaurants right on the beach that ae cheap and clean. Can't remember the names. Just check the washroom and you'll know if you are in the right place. (some are really bad and you don't want anything there unless it comes in a can or bottle.)
Especially with our stong CAD$ it is really attractive to go there these days. I don't care for Cancun, as I am not in to the party/bar scene, so I cannot really comment fairly. I have read that Cancun is a little better than 50% back to running normal. Some hotels have to be torn down and completely rebuilt.