i went last year in june and stayed at
Tilajari by Arenal. It was humid but beautiful and barely anyone was there. The hotel was awesome although a little far from the town. We did the hot springs, some hiking, and the canopy tour w/horseback riding and it was an amazing trip. I thought rainy season might mean lots of bugs or something, but all i ever saw were butterflies
the other great thing about the rainy season was that when we did the little day trips it was just the guide and us instead of like 30 people. The trips were in the morning mostly and the rain came in the afternoon so it didn't hinder us at all.