Since you may not have arrived yet, this may be of help. The place that should definitely go is Piedras Negras. It's a marked path up a volcanic rock hill that takes you to a cliff where you can spot tons of birds. The trailhead is at Playa Loberia. Blue-footed Boobies, Magnificent Frigatebirds, Swallow-tailed Gulls, Nazca Boobies, Brown Pelicans, etc.
If you're fluent in Spanish you might want to hail a taxi and go to Puerto Chino on the other side of the island. Just be sure to confirm with the driver that he'll pick you up for the trip back. I've seen Lava Gulls, Lava Herons (now Striated Herons I believe), various Finches, Brown Noddies, and even a Yellow Warbler over there. I was fortunate enough to be able to camp over there (camping requires a special permit which needs to be procured well beforehand, and you'll need to bring your own tent, etc) and got a Black-crowned Night Heron.
You can go up to Playa Punta Corola and bird at the beach for a bit, then hike the path to Mirador Cerro Tijeretas. Lots of Frigatebird and Pelican nesting in that area, along with whatever other avian live may be accessible in the foliage as you hike.
That's my $0.02, hope you have a great trip.