The Åland Islands reward slow exploration more than monument-ticking. The coastline is the main attraction — thousands of kilometres of granite shore, archipelago channels, and clear Baltic water. That said, a few specific sights stand out. In Mariehamn, the museum ship Pommern, the nautical museum, and the Åland Museum are all worth a visit. On the main island, Kastelholm is the highlight: substantial castle ruins, a pleasant open-air museum (free entry), and a prison museum occupy the same site.
The island of Kökar, reached by the inter-island ferries that are free for pedestrians, justifies at least one night's stay. The medieval church and the adjacent Franciscan chapel ruins can be explored on foot, though bike hire makes the island much more accessible. The Postal Museum at Eckerö on the main island is an unexpected gem.