Surrounded by rivers from three sides the medieval Old Town of Kaunas is located next to the confluence of two major rivers: Neris and Nemunas. This confluence, once important for trade, is still guarded by a tower of Kaunas castle (14th century) that you can climb witnessing minor exhibits.
Not far south from the tower lies the Rotušės (City Hall) square, the heart of the city and the location for any celebrations, be it the annual Hanseatic days that remember history of Kaunas as a trade center in era when Hanseatic Union dominated the Baltic trade, or the Christmas market, or military parades in the days gone-by.
In the center of the square stands the City Hall with a 53 m tall tower (1780). Nowadays it is used for registering civil marriages.
Marriages are also common in the five churches that surround the square so every summer you can see many newlywed couples here. The tallest spire is that of Accension Church that is almost universally reffered to as Vytautas church after the Grand Duke who commissioned it in 1399. This oldest building of Kaunas lies to the south of the Rotušės Square next to a gothic residential building known as Perkūnas house.
The churches of the square well represent every major pre-1800 architectural style. The southern flank is dominated by the Baroque Jesuit church (1720) and monastery with a summer terrace on top. West of the square behind historic palaces stands another large gothic church dedicated to Saint George (1490s) and a nicely repaired Renaissance priest seminary palace with its own Holy Trinity church (1634). Beyond them there is a Santakos (Confluence) park at the point where Lithuania's two largest rivers converge.
There are many other nice old houses and palaces near the square, some of them housing mediocre museums (dedicated to pharmacy, Lithuanian literature, ceramics and sport). Visit them if you are interested in a particular field (e.g. Sport museum may enthrall a collector with its plethora of historical medals and cups - but bore many others).
To the east of the square is the red brick Kaunas Cathedral that is the largest gothic building in Lithuania.
The main Vilniaus street beside the Cathedral leads eastwards from the Rotušės square towards the New Town. Today it is the most important street of Kaunas with many restaurants available in its old authentic buildings. Going east on Vilnius street you pass the dilapidated Baroque God's Body church facade (unfortunately its three towers and the interior was entirely destroyed by the Soviets and only a single room now is left for celebrating mass in what effectively became a multi-storey building).
After passing the underground passage there is the President's residence of interwar Lithuania with a small park. Now it hosts extensive year-long temporary exhibitions.
In a seemingly ordinary courtyard at the end of Vilniaus street a gothic Saint Gerthrude church is located. This meticulous small church dates to 1480.
Smaller and less busy streets parallel to the Vilniaus street are also a great place to stroll and watch the Old Town of Kaunas which suffered less of the post-war Soviet destruction than its counterpart in Vilnius and therefore is more intact. The Folk musical instrument museum located at Kurpių street has a nice array of Lithuanian traditional instruments (no on-demand audio however).
Nemunas embankment with its modest Lutheran church (1683), gothic buildings and views over Aleksotas is another alternative.
Wide Šv. Gertrūdos and Šauklių street at the north are where the lowland Old Town gives way to Žaliakalnis hill. 15th century St. Nicholas church and former Benedictine nunnery located there are however still a part of a downtown, its cute octagonal tower joining the ensemble of Kaunas gothic buildings.