I’ve been twice in August and it was fine. We had a great time at Aerenal - there was some rain, but mornings were clear and we fit in nice dry hikes, a float trip, etc. on our most recent trip, there were 2-3 days out of 10 where there was significant rain, and some short drenching storms other days, but we had a great time and managed outdoor activities even on the wetter days.. On our earlier trip, I remember a general pattern of sunny days with a period of afternoon thunderstorms.