I spent 2 weeks in costa rica last summer and I was all over the place. Spent some time on both coasts, as well as in the interior and mountains/rainforests etc. It was pretty humid but I don't remember ever being that uncomfortable at all. I'm from Connecticut though which already is decently humid. Not sure where you are from, but coming from colorado, wyoming, utah, or any other dry western state might be more of a shock. It also rains a lot which helps keep it cool. There were even some days where it got cool enough that I wore a long sleeve button down, but I think we got lucky with the weather.