Manuel Antonio is nice but I'd highly recommend against renting a car there as their addressing system is fucked up and it's practically impossible to find something without knowing the area. At least four years ago Google Maps wasn't any help either.
IMHO, I find Costa-Rica overrated, but I'm more of a luxury vacation kind of guy and Costa-Rica just seemed more geared towards the hiker/backpacker type. The beaches are also mostly rocky and ungroomed. The animal life is definitely fantastic, but that's maybe one day of your trip. For an inexpensive beach vacation with a latin tinge, I preferred Cuba.