The Galapagos are fabulous, even if there are not a lot of species. A good contrast is to do at least a little birding in mainland Ecuador first (you can now easily get to great places near Quito or Guayaquil), to experience the difficulty of seeing birds in forest, and then go to the Galapagos, where your biggest concern is that you wil step on one! Snorkeling was one thing that people don't expect, but be prepared to do it: you get to swim with sea lions, maybe penguins and cormorants, sea turtles, as well as colorful fish. (In the cold-water season, a short wet suit will be welcome.)
All boats take along trained guides, who will know all but the most obscure birds. You will have to convince them (and your fellow passengers) that you really want to go after all the finches, etc.
Visiting the islands need a week, based on a boat; there are a lot of agents who all book the same boats. We were quite happy on a 40-passenger ship, the Isabella II; I gather the 12-passenger ones, though appealing, are less comfortable. (Sails look romantic, but they all use their motors most of the time, to get between islands at night.)
"A Guide to the Birds of the Galapagos Islands" by Castro & Phillips is mediocre as a field guide; I've seen a book of photos of all animals (and plants?) that would be better.
There are advanatges to each season. But what would be tough would be going in a strong El Niño year (like maybe this year): the fish disappear without the cold water, and I've heard e.g. most of the newborn sea lions die of starvation. Could be fascinating, but not pleasant...