Tavira is often called the most attractive town in the Algarve. Straddling the Gilao river, connected by an old Roman bridge, it has the kind of quiet beauty that the bigger resorts have traded away. The rooftops are distinctive -- four-sided pyramid shapes called telhados de tesoura that you will not see elsewhere in Portugal.
The town has an unusual density of churches for its size. Santa Maria do Castelo (13th-18th centuries), Misericordia (Renaissance), and Sao Jose (Gothic and Manueline elements) are all worth a look. Along the Travessa de Dona Brites you will find medieval houses with Gothic windows and portals.
Tavira's beaches are on the Ilha de Tavira, a barrier island reached by ferry. The sand is wide and clean, the water warm, and the crowds manageable even in high season.