Reserva Natural Privada Yaxhá (tel. 2366-3411, www.yaxhanatural.org [1]) is a private, 407-hectare nature reserve on the southwest corner of Yaxhá Lagoon. Established as a site for scientific investigation, it has a newly completed biological station with dorms for up to 20 people.
The reserve is most easily accessible by boat from the Yaxhá archaeological site [2], though you can also get here via roads traversing neighboring properties. In addition to providing opportunities for scientific study in its biological station, there are nature trails for hiking as well as kayaks and mountain bikes.
The project is still in its early stages, so your best bet is to contact the reserve if you are planning a visit or simply want more information.
Links:
[1] http://www.yaxhanatural.org
[2] http://www.moon.com/destinations/guatemala/peten/the-maya-biosphere-reserve/yaxha-nakum-naranjo-natural-monument/yaxha