A few days birding around the Arenal Volcano area is worth the price if for nothing more than good access to several species of antbirds. In my opinion, accommodations at the Arenal Observatory Lodge have the best access to good habitat, trails and the occasional ant swarm, but there are several other places to stay around the volcano.
On my first day at the lodge, Bicolored Antbird, Ocellated Antbird and Thicket Antpitta were found near an ant swarm on the Saino Trail - one of two trails at the lodge worth your time. The other was the Waterfall Trail not far from the Saino Trail. Perhaps any trail would be fine if you found an ant swarm. Arenal is also good for Fasciated Tiger-Heron around some of the fast moving streams.
For an excellent bird guide at both Cano Negro and Arenal, I recommend Juan Diego Vargas (see my section on "Guides").
There are several good birding locations close by. At the Arenal Hanging Bridges I had my only observation of the uncommon White-fronted Nunbird along the hanging bridge trail system - which is close to most of the Arenal and Fortuna lodges. The birds are quite vocal and are frequently found in small groups. You are almost certain to get good looks at Broad-billed and Rufous Motmots at the same location.
There is a dirt road not far from most Arenal lodges that skirts part of Lake Arenal towards the south end - generally referred to as the Peninsula Road. With the help of a guide, this road produced White-throated Crake (grassy field with tape), Great Antshrike, Bare-crowned Antbird, Black-headed Tody-Flycatcher, and Yellow Tyrannulet. My guide also knew of a place for Keel-billed Motmot on this road - which is where most bird tours look for it. There have also been records of Sepia-capped Flycatcher on this road - a bird difficult to find in Costa Rica.