Tulum [1] is the first stop, of course, and for many people their main destination. From Tulum you can take day trips or short overnighters to Sian Ka’an Reserve [2] and Cobá archaeological site [3], both fascinating.
To venture any farther south you’ll probably want a rental car, as bus service grows infrequent. Mahahual [4] and Xcalak [5] are certainly worth savoring; despite their isolation, there’s plenty to do in both, including snorkeling, diving, kayaking, and fishing.
Laguna Bacalar [6] is worth a day or possibly two, to take a boat trip on the Caribbean-like water, swim in Cenote Azul [7], and visit the surprisingly good history museum in town.
Chetumal [8] is a logical stopover for those headed west toward the Río Bec region [9] or crossing into Belize [10], and it has a superb Maya museum [11].
Links:
[1] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/tulum-and-southern-quintana-roo/tulum
[2] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/tulum-and-southern-quintana-roo/sian-ka-biosphere-reserve
[3] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/tulum-and-southern-quintana-roo/coba/coba-archaeological-zon
[4] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/tulum-and-southern-quintana-roo/la-costa-maya/mahahual
[5] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/tulum-and-southern-quintana-roo/la-costa-maya/xcalak
[6] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/tulum-and-southern-quintana-roo/laguna-bacalar
[7] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/tulum-and-southern-quintana-roo/laguna-bacala/sights
[8] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/tulum-and-southern-quintana-roo/chetumal
[9] http://www.moon.com/destinations/cancun-the-yucatan/the-state-campeche/rio-bec-region
[10] http://www.moon.com/destinations/belize
[11] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/tulum-and-southern-quintana-roo/chetumal/sights