Walking around in Riga (you really have to pack good shoes here), what most surprises is the eclectic architecture. Notably in the Old Town, there is always a medieval highlight next to some Stalinist leftovers. This diversity in styles adds a lively atmosphere to the town. Generally, Riga is less open-air-museum-like than Talinn for example.
The main reason for inclusion is Riga's fine collection of Art Nouveau buildings. They can be found a few blocks north of the city center. There are several streets adorned with great specimens. The buildings by Mikhail Eisenstein are well restored and freshly painted.
Another feature of Riga is the 19th-century wooden buildings. They are less prominent than the Art Nouveau ones and less often renovated.