I spent a week in the Galapogos Islands back in 1999. I used Canon T90 with 24, 35-70, 80-200L, 300 f4L and 1.4 and 2x extenders. Few things to bear in mind:
1) Sometimes you almost fall over the wildlife walking around the islands, therefore you will need shorter lenses than for wildlife anywhere else.
2) IS would have been useful (didn't exist when I went, or at least not in FD system) for shooting from the inflatables on the way to/from boat to shore (sea lions, reptiles).
3) The one exception to generally very accomodating wildlife is Sally Lightfoot crabs, which do run away!
4) I found a hot-show mounted flash useful for low light early morning walk, but of course that was using film and we didn't have digital cameras with amazing high ISO performance then.
5) Generally, 70-300 will be most useful, although there are uses for longer lenses, eg. frigate birds in flight.
6) There are some great landscape opportunities - focal length is your choice
7) Something with macro/close-up capability willl be useful
7) Don't forget need to keep camera bag dry in the inflatables - I used black bin bag!
Have a good trip.