If you have 2 weeks you can do Arenal then Manuel Antonio and then the Osa Peninsula (Puerto Jimenez or Drake Bay). Both areas are different and very nice.

I will spend more time around Drakes...Osa Peninsula...mostly for the distances and hikes...so many great hiking options around Drakes...or boat tours...check at Matapalo beach...Casa Bambuu...great location...just in case you like to move from Drakes to Matapalo and out of Puerto Jiemenez...

roadadvisor

Much depends on your travel style as to where you will be more comfortable. Arenal is fabulous and a great choice. Manuel Antonio has the most to choose from in the way of wildlife, tours, activities, spas and good restaurants. Drake Bay is amazing (as well as all of Osa), and the wildlife and beaches are stunning. La Paloma Lodge, Drake Bay Wilderness Resort and Aguila de Osa are all great options. However, keep in mind that your meal choices will be extremely limited in Drake, and most meals will be served at your hotel, so that may make a difference. Also, they have great tours, but not as many as MA, so that may help you choose depending on what you like to do. All of those areas are fabulous, so Enjoy!