A few days at Arenal Observatory Lodge is worth the small price if for nothing more than good access to several species of antbirds. But the area is also good for several other species, including Fasciated Tiger-Heron around some of the fast moving streams. You are almost certain to get good looks at Broad-billed and Rufous Motmots at the Arenal Hanging Bridges
Bicolored Antbird, Ocellated Antbird and Thicket Antpitta were found near an ant swarm on the Saino Trail at Arenal Observatory Lodge - one of two trails there 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.
For an excellent bird guide at both Cano Negro and Arenal, I recommend Juan Diego Vargas (see my section on "Guides").
Arenal Volcano - Jim Peterson
Oscellated Antbird - Greg Lavaty
There is also a remote chance for White-fronted Nunbird along the Arenal Hanging Bridges 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.
Buff-throated Foliage-Gleaner - Greg Lavaty
Blue-and-white Swallow - Greg Lavaty