Arenal and Monteverde have highs around 70F and lows in the mid 50's over the next week. Manuel Antonio is 83/72 and San Jose is 80/65. If you're going up the Poas Volcano, expect it to be quite a bit cooler.
I'm heading down there on Friday and will bring long-sleeve shirts and a light jacket, and will layer up for a morning trip from Peace Lodge to Poas on Saturday morning. Plus two pair of shorts and short-sleeve shirts for a couple of beach days.
See you in the cloud forests...
http://www.costarica.com/Home/Weather