The drive to Arenal is 3-1/2 hours each way through some winding roads. I would not advise driving it at night.
We just returned Monday from a 6 night stay. There is a tour company on the beach outside the Marriott who is very good - Piko Travel they can arrange all sorts of tours. We did the monkey tour and it was awesome.
Best meals we had were Bambu (sushi) in the marina next to the Marriott and Graffiti in Jaco.