Yes, it is possible to take a day trip to either Arenal or Monteverde in a single day. However, these trips are long. It takes about 3 hours to get to both Arenal and Monteverde; this means you’ll spend at least 6 hours of your day on the road.
In many cases, we recommend spending at least 2 or 3 nights in either Arenal or Monteverde to get the best experience without feeling rushed. A typical Arenal + Guanacaste itinerary looks something like this.