The Marstal Søfartsmuseum is the kind of small-town museum that punches well above its weight. Ærø sent ships and sailors to every part of the world during the 19th century, and the museum's three floors are stuffed with the souvenirs they brought back: ship models in every scale, navigation charts, log books, ships in bottles, and oil paintings of vessels long since wrecked or scrapped. There is a particularly fine collection of figureheads.
The museum is best understood as an attic of an entire town's seafaring memory. Many of the items were donated by descendants of the captains and sailors whose homes still stand a few streets away. Allow at least an hour — more if you can read Danish.