I didn't use an ND filter and had no real problems - and both of my earlier trips pre-dated HDR!
I took a tripod but didn't use it very much. Most of the time on land you will be moving with a group and your setup time could be quite limited. Group size in Galapagos National Park is limited to 16 at a time but there are likely to be multiple groups at any given location. You will be restricted to the trail - even setting foot outside the trail for slightly better composition will be strongly discouraged and even criticized. We were not allowed to use flash in order to avoid stressing the wildlife - as if we tourists don't produce any stress.
See my website
to get a sense of what you might encounter as well as the results I managed using the equipment I had back then.