I'll try help you with the little experience I have about Cancun. I aws there a few years ago with a group of friends.
* Any extra activities thats you recommend doing, that is a 'Must Do' ??
Personally I recommend you the usual Chichen Itza and Xel-Ha. I've heard there's a new archeological spot that's amazing but I'm not sure about it's name or location (sorry)
* How much spending money do you suggest we should take? (We are all inclusive)
If you book your tours with the hotels agent each will cost you about 100-150$. Other expenses it depends on if you're willing to spend. Cancun is a small town with shops. Shops are always willing to help you leave money. And CAncun's nightlife also gives you a hand
