I spend a large part of my time in the tropics. For a short visit I would recommend you do absolutely nothing, seriously, it really isn't that challenging an environment.
Obviously don't get stuff soaking wet from rain, and if your room has AC then give your gear a short while on leaving the AC to let the condensation evaporate, better yet, just turn off your AC and enjoy the tropical air.
As for changing lenses, I have never, even when living in the tropics, taken any kind of precaution regarding humidity ingress. Even "weather sealed" equipment is not environmentally sealed, there is no point to changing lenses in a plastic bag. Not much point in storing them in them either. Relax and enjoy your trip and don't worry about your gear, I never have and it has spent years down there.
I dont think I could have put it any better myself!
Regardless of how weather-sealed you think your camera might be/not be a short trip to the tropics probably wont cause any damage. I'll be in Costa Rica myself in September (part of a 6 week hitch-hiking trip through Central America) and have used various iterations of the XXD series in every type of climate you can imagine, from -35 to 6000 metres altitude, the steamy tropics, sahara desert, Indian ocean and scaryest of all a British summer and never had any of them skip a beat. Cameras in my experience are a lot tougher than most people imagine. Saying that for my upcoming trip I will be taking one or two small precautions and these are mostly about how I carry and storing the body and spare lenses whilst I'm hiking etc.
As I'm sure you know Sept isn't exactly dry season in Central America so rather than fretting about changing a lens when the sun is out you'd be better investing in a few good canoe dry bags for storing gear in whilst its bucketing it down outside and your gear is in your rucksack etc. My last trip to Central America (10 years ago now) as a fresh faced 18 year old was a real eye opener as to just how spectacular rain can be. I heard it best described as "the sea trying to reclaim the land by airdrop". I tend to find that when it is throwing it down and I'm out and about I'm not really in a taking pictures mood and want my camera to be tucked up snug and dry in my rucksack leaving my hands free to string hammocks/tarpaulins etc etc. In this type of situation a good canoe dry bag is your best friend. When strolling around Patagonia I also found that by clipping a macro lens in its dry bag to my rucksack strap I could have it quickly accessible whilst not in the way of hiking type movements.
It's worth pointing out that I use the canoe dry bags because I dont take a camera bag of any type. I'm there to do 3 day - 2 week hikes and camera bags are just excess weight and hassle on such a trip. for my gear I take 3-4 dry bags (5 if I take a flash).
For the upcoming trip the three of us will be loaded up pretty well with camera gear: I'll be taking a 5d mk2 and 4-5 lenses, my brother a 40D and 2 lenses my girlfriend a Powershot SX280 HS and between us we'll also be packing 2 GoPro version 3 black editions. The GoPros will obviously be fine and for the rest its canoe dry bags with a few silica gel sachets thrown in to store gear in whilst monkeying around in the rainforest.
Oh I forgot 1 last tip. when using canoe dry bags its possible to put a jumper/fleece/walking socks in there too as cushioning and then not properly vacuum seal them so you also get a little air cushioning too. Handy if your stuffing gear in an overloaded bag!
Look forward to seeing your photos when you return and hope you have a truly fantastic trip.
Ps. hopes this makes sense I'm just back from a Romanian beer festival.....
Well said. Rain forest doesn't mean always rain, just humid, and rainy season means usually pouring rain for 1 hour in the afternoon. Best look up the yearly weather forecast for your exact destination, it can vary a bit depending on winds, mountains etc.
As to humidity, if you are there for a short time, it should not be an issue. In air conditioned rooms it's no issue at all, the air is dryer there from the air-conditioning. Transitions from cold to hot - humid air are something you have to be careful of, as has been pointed out, i.e. going from a cold airplane, or cold room outside into humid air, you will get condensation on your gear. Let the gear warm up. If not, you can take scenic pictures with your front lens fogged up.