CESiaK (Hwy. 307 just south of the Tulum ruins turnoff, tel. 984/871-2499, www.cesiak.org, 9 a.m.–2 p.m. and 4–8 p.m. daily) is a longstanding nonprofit group offering excellent tours of the Sian Ka’an Biosphere, a 1.3-million-acre reserve of coastal and mangrove forests and wetlands, with pristine coral reefs and a huge variety of flora and fauna.

The most popular is an all-day excursion (US$77 pp) that includes visiting coastal dunes and taking a motorboat tour of the several lagoons, plus stops to see a small Maya ruin, float down a mangrove canal, and snorkel in a cenote. The sunset bird-watching tour (US$77 pp, Dec.–May only) includes a motorboat tour of three bird-rich lagoons and a stop at the aptly named San Miguel Bird Island.

Guided bird-watching tours in kayaks are also available (US$50, 3 hours), or you can rent kayaks (US$22/33 single/double, 3 hours) and explore the lagoon yourself. Most tours include hotel pickup, lunch or dinner, and a bilingual guide. CESiaK also has well-recommended lodging just inside the reserve.

Community Tours Sian Ka’an (Av. Tulum btwn Calles Orion and Centauro, tel. 984/871-2202, www.siankaantours.org, 7 a.m.–9 p.m. daily) is an excellent community-run agency offering a variety of Sian Ka’an tours, such as the Muyil route (US$99 pp, 8 hours), which begins with a visit to Muyil archaeological zone, then a boat tour of Muyil and Chunyaxche lagoons, including a chance to jump in and float down a long mangrove-edged canal; “Mayaking” in Sian Ka’an (US$45pp, 3 hours), a kayaking tour through the lagoons and mangroves, with an emphasis on spotting birds and aquatic life; birding (US$75 pp, 3 hours) with an experienced local guide; and fly-fishing in Ascension Bay (US$550 pp, 8 hours) at three different flats.

Community Tours is the product of a unique collaboration with RARE, an environmental group that helps local cooperatives create self-sufficient ecotourism projects.

If your budget permits, another good option is Visit Sian Ka’an (Visitor Center, Sian Ka’an Biosphere Reserve, cell. tel. 984/118-2555, www.visitsiankaan.com). This nonprofit organization offers a variety of private tours of the Biosphere Reserve but also can customize tours to your needs and interests.