A day trip to Arenal is possible but it is a long day. Price will vary by the activities that you choose to do while there.
You don't mention any ages, but other suggestions would be Buena Vista for the combo tour, the combo at Canyon de la Vieja with tubing or rafting, definitely the Palo Verde Crocodile Safari for nature, a catamaran sunset cruise, maybe ATVs to see the area.
So many choices - you won't be bored.
You can do a day tour to Arenal but is it really a long day over 4 hours on the road each way. Other tours are the Palo Verde Boat tour, the Rincon de La Vieja Volcano area where you can do hot srpings, ziplines, tubing, Whitewater rafting at the Colorado River etc.
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