I'd stay in/near Cancun. There are many other beautiful spots to visit but with the latest info on the cartels fighting I wouldn't go near any border town or anyplace close to a border town. Many of my friends in San Diego who border hop a lot won't go right now.
The tourist areas are fine, especially down south. We've stayed in Cancun (touristy, but easy to navigate esp with the bus system and tours based there). Puerto Morales just south of Cancun...it has an alligator farm. Whohoo!. Then near Tulum further south.
Seeing Xel-Ha, Tulum, Chichen Itza, and the underground/underwater caves are a must.
You may look at going to Belize instead. It's been independent from Britain for over 20 years now. Consequently everyone speaks English and it's the law is based on Common law, just like the US . It's just a few hundred miles south so the diving, beaches, weather, prices etc are all the same as Southern Mexico. Ambergris Cayo was amazing, and there are no cars and San Pedro is the town that Madonna sang about. The accessibility from Houston is the same as for Mexico.
Happy vacationing!!!