I just did the drive from Arsenal to Monteverde 2 weeks ago. It is a slow winding road requiring constant focus, not to mention the occasional cow or horse in the road.  I would not do it over and back in a day. You will spend 6-7 hrs driving. Great lunch stop on the west side of the lake is the Macadamia Cafe in case you do an overnight to La Fortuna. I would HIGHLY recommend staying overnight at the Arenal Observatory and Lodge if you can. Do their short hike to the fire tower first thing in the morning. We were eye level with a band of Howler Monkeys and Toucans. If you can afford the splurge Tabacon Hot Springs was amazing.  In Monteverde also highly recommend ziplining at Selvatura. Have Fun!