Cancun was built to be a touristic place, so it has many commodities for all tastes: relaxed, family, adventure, nightlife, etc, etc. Just about anything you can imagine, you can get here. There's actually so much to do that you probably won't be able to do it all in a week or even two!
You can also visit the town, the bus will take you there, so you can take a closer look at the Mexican lifestyle of the people in Cancun. Some say you don't know a place until you get out and talk to the locals - well, you can try that as well.
Quick Tips:
If you are buying something, bargain, they are expecting you to!! Also corruption is at a very high peak there, you can get away with almost anything there for the right price, so if you really want to do something, and someone tells you you can't, just offer some money and you'll probably be able to. This is not nice at all, but that's just the way it is.
And by the way, don't piss on the street, you'll get arrested for that (or you can pay the cops not to take you in, and there are a LOT of cops out there).
Best Way To Get Around:
The bus is just great, it's and it will take you anywhere you want to go. If you are unsure where is the place that you want to go, just ask the driver to tell you where you have to get off, they are very nice and will let you know.
If you are taking a cab, ask the price of the ride first, and remember - bargain! And if you don't have a ride to the hotel when you get there, don't take those taxi vans, you'll be better off in a regular cab. But if you like to walk, you can also walk just about everywhere, it's safe at any time as there are many cops out there 24/7.