We will plan to go here on Saturday! It is a 20 minute walk from the resort. Steve and I will likely book the ziplining on their website after we get to Costa Rica as we will want to make sure they aren't calling for rain on Saturday. If you want to do ATV tours you can book in advance online as well.
About a 3 hour drive from the Riu Guanacaste resort. Could be a day trip or somewhere to spend a couple days before flying back out of Liberia airport
Tamarindo is about an hour drive from the Riu Guanacaste resort. Cute little beach surf town, I have been here twice. Last time I think there might have been more expats here instead of locals though, so depends on the kind of experience you are looking for.
Flamingo Beach is only about a 30 minute drive from the Riu Guanacaste resort. I have never been but have met other travelers that said this is a great area to visit.