San Marino's headline attractions are its Tre Castelli—medieval architectural wonders that appear on every coat of arms in the city. Starting in the center of town, walk a few hundred yards past the trinket shops, along a paved cliff-top ridge, from the 10th-century Rocca della Guaita to the 13th-centuury Rocca della Cesta (containing a museum of ancient weapons that's worthwhile mostly for the views from its terraces and turrets), and finally to the 14th-century Rocca Montale (closed to the public), the most remote of the castles.
Every step of the way affords spectacular views of Romagna and the Adriatic—it's said that on a clear day you can see Croatia. The walk makes for a good day's exercise, but is by no means arduous. Even if you arrive after visiting hours, it's supremely rewarding.
San Marino, Emilia-Romagna, 47890, Italy