The Sandelin Museum is housed in an elegant 18th-century mansion and is one of the best small decorative arts museums in northern France. The collections were assembled by the Viscountess of Fruges in the early 19th century and include Flemish and Dutch paintings, medieval sculpture, Saint-Omer faïence (the town had a flourishing pottery tradition), and an exceptional collection of Flemish tin-glazed earthenware and objets d'art.
The museum's real surprise is its medieval wing, which holds some remarkable Gothic and Renaissance pieces accumulated from the suppressed religious houses of the region after the Revolution. Given how quiet and unhurried the place typically is, this is somewhere you can spend an hour with serious works of art without the usual jostling. The building itself, with its formal French garden, is worth a visit on its own.