Cesis is the most appealing town in the Gauja Valley and arguably the best-preserved medieval settlement in the Baltics. It's about 90 minutes from Riga by train and makes an easy day trip, though an overnight stay lets you see it more slowly.
The town's centrepiece is Cesis Castle (Cēsu pils), a 13th-century fortress of the Livonian Order. The ruins are substantial and atmospheric; the visit includes a museum and a medieval tower you can climb with lanterns provided, since it has no electric lighting. Beside the ruins stands a 19th-century manor house (Jaunā pils) that now serves as the local history museum.
The old town around the castle is quiet and well-kept, with wooden and stone buildings lining cobbled streets. St John's Church (Jāņa baznīca) is a large Gothic church from the 13th century with a tower offering views over the valley and surrounding forests. The local brewery, Cēsu Alus, has been operating since 1590, making it one of the oldest in Northern Europe; the beer is worth trying.
The Gauja Valley below the castle has excellent walking trails through forested sandstone riverbanks. The town has historical resonance for Latvians: the Battle of Cesis in 1919, which repelled the German Landeswehr and secured the future of the independent Latvian republic, was fought nearby. The Cēsis Concert Hall hosts chamber music and a summer festival.