The all-inclusive resorts are all clustered along the north pacific coast with only a couple of exceptions. They represent a tiny fraction of lodging options in Costa Rica and they are not located in areas with plenty of wildlife but in the drier corner of the country. You may be able to take daytrips whitewater rafting from pacific beaches but don't count on it as rafting can be limited at the end of the dry season (April) due to water levels in the smaller rivers near the coast.
Might make more sense to head inland to Arenal first for 3-4 days for a chance to enjoy whitewater rafting, ziplining, hotsprings and wildlife before heading to an AI on the beach. Arenal is the adventure capital of Costa Rica and generally much more lush than the north pacific coast.
Might want to check out an earlier forum post about AIs which include a list of options on the last page at tripadvisor.com/ShowTopic-g291982-i813-k4639…
Edited: 11:48 am, July 10, 2013