Hi E&C,
For the 3 nights after Manuel Antonio you could check out:
The Costa Ballena – Area about 45 minutes to 1 hour south of Manuel Antonio but less developed.
Drake Bay – Very remote but a great place to see wildlife (you would park the car for a few days).
Puerto Jimenez – Also remote, but you would drive. Check back or subscribe to our blog, we’ll be writing a post about this soon.
Jaco/Tarcoles – Jaco is built up but there is a lot of things to do and many great restaurants. Tarcoles is nearby and has Carara National Park, which is great for birding.

Hope this gives you some ideas. Have a great trip!