Cyprian said:
I recommend paying for GPS with your rental car. Addresses there aren’t typical, and using local names makes navigation easier.
Consider downloading Waze on your phone for driving directions.
If you have prescriptions, consider stocking up there; local pharmacies can help with many conditions.
Visit La Fortuna for volcano hikes and hot springs, then go to Manuel Antonio for beautiful beaches. You might enjoy the unicycle, but be wary of hilly roads. For ideas, this Costa Rica directory link helped me a ton.
The area around Arenal Volcano has lots to offer, including the Hanging Bridges Park. If possible, hire a guide to enhance your experience. Discover unique wildlife and make sure to check out La Fortuna Waterfall. Bring water shoes for swimming! Several hot springs can also be found nearby. Local food is delicious, so consider exploring roadside eateries.
It sounds like an amazing opportunity and trip.