Well you're going during the rainy season, so expect it to be rainy. The interior will be cooler and wetter than the coast. The weather was pretty consistent the whole time I was there. In the mountains it would rain once in the morning, once in the evening, then during the night. On the coast it only rained in the evening.
Use this to see what the weather will be like in any particular area: http://costarica.com/weather/
The hardest thing to get used to down there is the sun schedule. Say goodbye to 9 o'clock summer sunshine. You'll get sun from 6 to 6 down there, so make use of it.
And here's some bugs for you: