3 days is a really short trip but if you’re strategic, you can definitely make the most of it. For starters, we recommend renting a car so that you have the most flexibility to get around. Signage isn’t great in CR but if you get a GPS with the rental or use an app with maps on your smartphone (Waze works great here), you’ll be fine, especially around the Liberia area which is well traveled. Make sure to check out our rental car discount through Adobe to maybe save some money. They have an office at Liberia airport and can pick you up there, making it really easy. There’s more info on our discount here.
La Fortuna is a little far with your timeframe so we’d suggest Rincon de la Vieja volcano instead, which is much closer (follow links for more info). There’s also a great waterfall nearby called Llanos de Cortes. A couple of nice, quiet beaches to consider for an overnight are Playa Flamingo, Playa Avellanas, Playa Brasilito/Playa Conchal, and Playa Hermosa- they’re all a little different so depends what you’re looking for but are great choices. Best of luck planning!