Hi Amanda,
The roads from Manuel Antonio to Rincon de la Vieja are mostly highway so a compact car that is not 4×4 will be fine. Getting to Rincon, you should go to the Las Pailas ranger station since the road is accessible with a regular car. Read our post Rincon de la Vieja National Park for more details.
As for places to visit along the way, you could stop at the Tarcoles River Bridge (north of Jaco) for a few minutes to see the huge crocodiles living below. Then as you get closer to Guanacaste, there is the Llanos de Cortez waterfall and/or the Las Pumas Wildlife Center. Both are south of Liberia and make for a great 1-2 hour stop.