Fingal's Cave looks like something a set designer built, floor-to-ceiling hexagonal basalt columns rising from the sea, leading into a cathedral-sized cavern where the waves echo. Mendelssohn heard it and went home to write an overture. You'll understand why.
Access is by boat from Oban or Mull, it's a journey, which is part of the point. Puffins nest on the island in summer. Come prepared for weather, come with a fully charged phone.