If you've never been to Cancun before you should know what to expect before you get there. Mexico's second highest source of income (after oil exportation) is tourism. The government is not foolish. They recognize the power of the dollar and have carved Cancun out of the jungle. It was created solely as a resort and vacation destination in the last half of the 20th century. Do not expect endless deserted beaches. Do not expect a huge city. The beaches are not overly crowded (at least in January) and the small city is there is of necessity to support the natives who work at the hotels, resorts, and restaurants that you will visit. The water in Cancun hotels and resorts is safe as is food in the hotels, resorts, and restaurants.
The Gulf of Mexico is the most amazing shade of azure you're ever seen. With a good camera, you could get postcard quality shots. The beach is not as large as it once was. Hurricanes in the past decade have taken a lot of the white sand out to sea but there is still plenty of beach for those who seek it.
The nightlife is there if you want it from dark to dawn. Activities abound for young and old, but handicap accessibility and OSHA regulations are concepts very foreign to this culture. Curbs are steep, sidewalks are rough, and sometimes cobblestone. There are few museums in Cancun but archaeological sites abound in the vicinity. Hire a private guide, take a tour, or strike out on your own if you're the more adventurous type.
Whether at a hotel or resort, Cancun can be an ideal place to shop a little and then drop a little. Shop in the morning while it's cool and then laze by the pool all afternoon. In the winter months, it does get cool in the evening so don't plan to swim all night long.