I was in Costa Rica and a rainforest last year and I suggest bug repellant for sure, hiking boots because some of the ground surfaces can be slippery, I didn't need any rain gear because it was warm enough and I don't melt. I just wore a long sleeve cotton shirt and shorts.
I had a great experience. Hope you do too.
What excursion are you doing? We just got back from there & did the Zip Lining. We took cheap rain ponchos that we could pack & carry easily and just throw away when we were done - we didn't need them but I don't regret having them there just in case. Bug Spray - I took some and our guide had some - word of advice (ok a few words of advice) - put it all over you - I put it on my bare arms but not my legs because I had wind pants on - my legs got eaten alive! I didn't even know it until I was back on the ship. Yet my husband didn't get a bite - it was weird.