A couple of days should be enough for visiting the Arenal volcano. Not far from here there is another National Park you should not miss, the Monteverde.
You can get to Monteverde by road or by boat. The boat tour will take you three or five hours. You will get a jeep in Arenal, cross the beautiful Arenal Lake, and get to Monteverde in a minivan. This is the best option if you are relying on public transport.
If you rented a car, you should take the road around Lake Arenal. The ride should take you between four and six hours.