I love Arenal and if you can, you should spend one night there. It is one of my favorite areas in the country, among many. I love the zip line in Arenal that goes over the La Fortuna waterfall and another waterfall too, can't recall the name of it. The canopy tour; www.arenalmundoaventura.com/ing/canopy.html
You could do a guided hike through the hanging bridges in Arenal and spot more wildlife than in Rincon, but Rincon, near Coco, is beautiful too. If you are really interested in seeing a sloth, you should try in Arenal.
I have done several canopy tours and that is my favorite for the scenery and crew. Second favorite is the Sky Trek in Monteverde for the height and length of the cables.
There is horseback riding near Playa del Coco at Haras del Mar. You can choose from many dive shops for scuba out of Coco too.
Have a blast!