We went to Chiapa de Corzo to wander around the square and to take a boat tour of Sumidero Canyon. We also took some time to go into the market and eat some delicious food. Choose any stall that is busy but know that a) most stalls only serve one kind of food (tacos, tamales, caldos, etc.) and that b) Spanish is a must. But you'll save yourself a lot of money by eating in the market than going to the restaurants near the river. A plate of three tacos cost about $20 MXN whereas the menu del dia at the restaurants on the river cost about $65 MXN.
The tour of the canyon is well worth the $160 MXN and takes about two hours but know that information is only given in Spanish. It seems as though there is only one tour company that runs these tours. We found a booth on the square but you can also go directly to the river to purchase tickets.
While in town, we saw a ruin of some sort up on a hill near the church (which is on another hill) overlooking Chiapa de Corzo. It's well worth the short walk uphill for the ruin and the views of the area.
To get to Chiapa de Corzo from Tuxtla, you must take an autobus to an area called Soriano (I think it's one of the bus depots but I'm not sure). At Soriano, on the roadside, there will be colectivo drivers hollering for passengers going to different destinations that are close-by. The cost was $12 MXN each way.