Spent a month in Costa Rica around this time a few years ago. It is the rainy season there but considering how long we went for, it didn't rain all that often. Most of the time its only for a couple of hours, definitely no shortage of sun.
I honestly think I'll retire there one day, its an amazing country. LOcals are super nice, endless hiking in rainforests, fresh fruit, beautiful scenery, surfing and stellar beaches. You'll see so much wildlife you'll think your on the Discovery channel. I found it helped quite a bit to know some spanish, especially since we rented a car for the month and drove from place to place, kind of funny when you ask for directions and show the person your map and they point to a place several inches away from where you think you are. Doesn't help that there are a bunch of towns that are named the same.
Bar none it was the best trip I've had so far, would go back there in a heart beat. If you do go I can give you more beta on where to go to and where to avoid.