Hi Vicky and Lindsay,
You’ll need four-wheel to get to and from Monteverde. The road there is rough dirt for a lot of the trip so it’s best to do with an SUV with four wheel drive, especially since you’ll be traveling during the beginning of the rainy season. As for other destinations, you could spend a couple of nights in the Rincon de la Vieja area after you visit Monteverde on the way to Guanacaste. This is a charming, rural area made up mostly of small towns. The main attraction in the area is the national park, which has interesting volcanic features like steaming fumeroles, and there’s also horseback riding and you can bathe in volcanic mud. There’s more info on the park in this post.