Zedrush, is it the Riu you're staying in? If so its awesome, stayed there a couple of years ago. If Henry still works in the bar near the pool (the one that's open late, near the stage), slip him a tip on the first night and he'll well look after you. He also makes a mean Mojito!
Definately also recommend Chichen Itza and the swimming with dolphins - plus the missus will love that
The scuba diving in Mexico is incredible, with great reefs and top fish.
iirc we spent about £400 in 10days which was mostly on trips, all in dollars though. My Sis is out there all the time as she is works on the airlines and she always says to use the dollar - it's worth more to the locals. Cancun isn't really a visit, just looks like an American city tbh. Had a couple of nights out there and not too bad but not great.
Couldn't go to Xel-Ha last time I was out there as had a broken wrist but my mates that went said it was stunning.
Last time I went I stayed at the Moon Palace which was incredible but nearer to Cancun than Playa Car, I reckoned that the sea was much clearer and the beach nicer at Playa Car.
Sure you'll have a wicked time mate I absolutely love the place!!