I visited the Pantanal on a 4-day tour from Cuiabá. Not to the protected zone, but to another nice part where there are lots of animals. The tour included all the local fun, like walking, horse-riding, bird spotting and piranha fishing.
Although at the end of October, it was still very dry in this area. Fortunately, because of that, there were not many mosquitos. The wildlife was still abundant though. We saw a tapir, otters, capybara's and alligators.
I think a bird spotter will find real joy in the Pantanal. In almost every tree you can see a wonderful example of the local birds here. To the more common visitor (like I am), the Tuiuiu (a 1.40 m. high creature) is amazing to watch.