1) Do the Palo Verde River Tour. It is a half day experience with guides pointing out birds, monkeys, and water life. If the RIU tour desk cannot help you, try info@costaricawelcomecenters.com.
You will need to get the map to drive yourself (1 hour) from the RIU to Ortiz, a small town east of Filadelfia. It is where you meet the tour.
Tour $46/person, $56 with lunch.
2) You could also take the Monkey Trail Road to Potrero/Flamingo for a day trip of your own. Rent a mid-size 4WD car to be sure if there are rough spots. There is a zipline experience on the way, called the Congo Trail.
Once through the Monkey Trail Road, there are several beautiful beaches, ocean kayaking, sunset cruises, and great spots for lunch or dinner. You can take smooth paved roads back to the RIU in an hour.
The Potrero office of the Welcome Center system can help you. Email maggie for help - maggie@costaricawelcomecenters.com
Have a great time!