I just traveled to CR in December with a friend and my 2 teenage daughters. We used Desafio Adventures for our transportation from airport/La Fortuna and also for all our activities in the Arenal area. Everyone was friendly and efficient and we had no problems. All our tours/activities were awesome. I can't recommend a lot in the way of sightseeing as we went mainly for the adventure activities. But if that is of interest to you ziplining, whitewater rafting, hiking and canyoning were a blast. They also have horseback riding, SUP, kayaking, etc. so there is plenty to keep you busy.
We rented a car for the 2nd half of the trip to go down to Playa Hermosa Guanacaste (totally different area than the one near Jaco). Driving was not difficult, although roads are somewhat winding. Some areas (mostly up near the Arenal area) have some big potholes, but not as bad as we had imagined. There are some buses, but I would recommend either private transfers as the price is the same whether there are 2, 4 or 6 pax, so it would be fairly reasonable with 4, or renting a car, which would be even more reasonable. I did look at the bus schedule, but there was not a lot of service for our route.
CR is a beautiful country, with wonderful people who go out of their way to help you. I'm sure you will have a great time!