Traveling to Costa Rica was a blast. Again, first thing is to time it right. Everyone says do not go during the rainy season. The rainy season is when I want to go. Less people more time for fun and it is cheaper. According to most people that lived there and what I heard is that they did not know there was a rainy season. It rains all the time.
I would suggest Septemeber. I have only been there once, but we had very little rain and there were hardly and tourists.