Sounds like a fun trip. I've been to Corcovado. We were not backpacking, but we did hike into the park and take a lot of day hikes. It has a high concentration of large animals (tapers, howler monkeys, spider monkeys, lots of scarlet macaws) so that is good. It was the dry season, in January, so I'm not sure what May is like. We also visited a dry forest (Palo Verde) and a cloud forest (San Luis), and of the three my favorite by far was the cloud forest. Again this was the dry season so that may have been why the cloud forests mist and cool air appealed to me...
There is plenty to do in Corcovado for 5 days though. Where else are you going?