Without a doubt, the biggest thing we did on Arran was exploring the standing stones at Machrie Moor. Since we went in December, which is definitely off-peak time to visit the island, there wasn't much open.
We left from Ardrossen and landed in Brodick around midmorning before heading across the island to see Machrie Moor. There was not much to eat on the western coast (and by not mean, I mean literally nothing was open. In an attempt to maximize our time on the island and see as much as possible, we looped around the northern part of the island, stopping in Lochranza for a bite to eat.
Lochranza is a small village of around 200 people. We ate lunch/dinner in the distillery as it was the only place to eat in town and one of the ladies was talking about life in the town, and said everyone there knew everyone else. She said that her parrot needed food and the ferry to the mainland wasn't running and she posted on the Facebook page she needed parrot food, and one of her neighbors braved the elements and brought her food. And according to Wikipedia, this town has the least daylight hours in all of the UK, due to it's location in a north-facing glen. It was an adorably friendly town, but I can only imagine how lonely it can be during the long, dark winter months!
After a hearty meal of soup and sandwiches, we popped over to Lochranza castle in the last hours of daylight to snap a couple of pictures before catching the ferry home from Brodick. We were at the castle around 4ish and it was already getting quite dark. On the way home, we drove by Goatfell, which is the large mountain on the island, saw plenty of red deer on the sides of the road.
If you're on Arran there is plenty to see and do, no matter what time of year you go. When we go back, we'll visit Brodick Castle, located in Brodick, and we'll explore the southern half the island, since we didn't make it down there!