Cancun has many excursions to offer. One that I personally think you should not skip, is an excursion to the ruins. There are a couple different ruins to pick from, Tulum, Coba, Chichen Itza or a combination.  
I decided to go to Tulum and Coba. This trip was all day, around 12 hours. Tulum was small but beautiful, right next to the ocean. Coba was a unique experience. Once at the site you had to walk about a mile to reach the ruins. There were bikes to rent as well. Once at the ruins you had the option to climb to the top. Coba is steep, slippery, and crowded. Coba stands above the trees. Once you're at the top you feel quite literally on top of the world. It was definitely a once in a lifetime experience.