Yaroslavl — the capital is a UNESCO World Heritage Site, nearly 1,000 years old, full of major cultural monuments—you haven't seen the Golden Ring if you haven't seen Yaroslavl
Borisoglebsky — a small town near Rostov, which is often visited for its Monastery of Boris and Gleb
Myshkin — a small town on the river cruise route with a pretty Cathedral of the Assumption and other 19th century architecture
Pereslavl-Zalessky — a small city full of important monasteries, cathedrals, and even the church where Alexander Nevsky was baptized
Rostov Veliky — this town has way more than its fair share of Russian culture, with an inspired kremlin and several of Russia's most important monasteries
Rybinsk — the second largest regional city was formerly a major trade center on the Volga; Rybinsk is home to Rybinsk dam (which created one of the world's largest man-made lakes), a prominent cathedral, and a particularly impressive Roman Catholic cathedral (a testament to the city's historical trade importance)
Tutayev — a large town containing many old churches along the riverside of the Volga
Uglich — a 1,000 year old town with a kremlin and the Alexeyevsky and Resurrection Monasteries
Pereslavsky National Park
Yaroslavl Oblast is full of rather magnificent destinations for onion dome loving travelers on the Golden Ring circuit. The region has benefited from trade throughout Russia's history as it lies on the Volga River and is relatively close to both Moscow and Saint Petersburg. Under Stalin's rule, a massive dam was built at Rybinsk, creating the Rybinsk Reservoir, which is about the size of Brunei, and which flooded some of the region's most important religious sites, especially at the town of Mologda, where 300 residents refused to leave and drowned.
English and possibly other languages are often spoken at major tourist sites, but in the rest of the regions, Russian may be the only language that you encounter.
Trains arrive from both Saint Petersburg and Moscow (~4 hours) to Yaroslavl, and from Moscow to Rostov Veliky (3 hours).