Varna is Bulgaria's third-largest city and the de facto capital of the Black Sea coast. Unlike the purpose-built resort towns to the north and south, Varna is a real city — with a functioning port, a good archaeological museum, a lively pedestrianised centre, and a large seaside park that fills with Bulgarians on summer evenings.
The Archaeological Museum is one of the best in Bulgaria and home to the Gold of Varna — a collection of Chalcolithic gold artefacts discovered in a necropolis in 1972, dating to around 4,500 BC. It's one of the oldest worked gold collections in the world, and the display is excellent.
The sea garden (Morska Gradina) runs along the coast for several kilometres — shaded paths, open-air cafes, and access to the city beaches. The beaches in Varna itself are crowded in summer but functional; better beaches are to the north (Albena, Golden Sands) or south.
Varna has good transport connections: an airport with direct flights to much of Europe, rail and bus links to Sofia (6–7 hours), and ferries to Odessa. It makes a better base than the resort towns if you want to explore the northern coast.