Our base was Los Lagos resort, which provided views of Arenal from a variety of vantage points. (My personal favorite was from the swim-up bar in the naturally-heated pool.)
These photos were taken from my hotel room. It was incredible to walk outside and see the volcano, whether it was partially hidden by clouds or completely clear against a bright blue sky.
As impressive as Arenal was during the day, the real show was at night. What look like streaks of red lightning travel down the side of the volcano. It's actually red hot boulders bouncing down, leaving a trail of sparks behind. With binoculars, you can actually see the boulders as they bounce along -- it was so cool! Sadly, my little camera wasn't really up to the task of night-time volcano photography.