The castles at Hohenschwangau, four kilometres east of town, are the reason most visitors come. Neuschwanstein is the postcard one — the Disney prototype, the fairy-tale silhouette on the crag — and Hohenschwangau below it is the older, plainer, and arguably more interesting royal family seat where Ludwig II grew up. Tickets must be booked online days in advance; both castles can be combined in a single half-day.
Don't underestimate Füssen itself, which has a handsome Altstadt of painted facades, the baroque St. Mang's Abbey on the Lech river, and the Hohes Schloss (High Castle) with its remarkable trompe-l'oeil painted courtyard. The Forggensee lake stretches north and the Tegelberg mountain cable car east — both worth a half-day each for hiking or for the view.