Cano Negro is a marshy wetland with water levels that are highly variable during certain seasons. It is a good place for wetland birds like Bare-throated Tiger-Heron, Sungrebe, Green Ibis, Northern Jacana, and Snail Kite. Occasionally, Great Potoo's are seen from the boat tours. Recently Jabiru have been observed nesting at Cano Negro - but the Jabiru is easier seen in Palo Verde National Park.
Unfortunately, there is no trail system into any wooded habitat. Most of your birding will be done by walking the small roads or by boat.
Many tours to Cano Negro are offered from places around Arenal Volcano as well as places near the Tenorio Volcano like Rio Celeste Lodge. One's options are better with a good bird guide. I personally know people who have picked up true rarities like the irregular Gray-breasted Crake and Green-and-Rufous Kingfisher with a strong bird guide.
I have visited Cano Negro in both the wet season and the dry season. Despite what you may hear to the contrary, both times are good for birds. In the wet season, one doesn't have the volume of egrets, shorebirds, or ducks, but the diversity of resident birds is about the same. During the wet season, water spills out all over the pastureland, and from the boat one can usually see wide expanses of wetland. In the dry season, the boats exclusively follow the Rio Frio River channel where the river banks sometimes rise above your general eye level. It's really a toss-up in terms of bird sightings though. In all seasons, small boats for hire were always available.
The boat trip is about the only place for Nicaraguan Grackle, observed in a few small grassy patches on the boat trip. The area is also a good place in Costa Rica for Black-collared Hawk and Lesser Yellow-headed Vulture (flying very low over the wetlands). Spot-breasted Wrens can be found along the dirt roads surrounding the lake, and occasionally Green Ibis and Boat-billed Herons are seen from the boat.