The Novgorod Kremlin is located on the Bank of the Volkhov river. This is the oldest Kremlin in Russia, since Russian statehood was born under Prince Rurik in Veliky Novgorod. St. Sophia Cathedral, built in 1045, is preserved here. This is the oldest temple in Russia. It will take at least half a day to see the entire Kremlin ensemble. You should definitely visit the Vladychnaya Plata Museum, although the Novgorod Historical Museum is also very interesting.
Walk along the fortress wall of the Kremlin, see the monument to the Millennium of Russia. Then cross the bridge over the Volkhov river and go to Yaroslav's Courtyard. It is a Trade and Veche Novgorod area, where 12th and 13th century it built several churches and St. Nicholas Cathedral.
After visiting Yaroslav's Courtyard, you should go even further to the Church of the Transfiguration of the Saviour. It is of exceptional cultural value for Russia, because only it has preserved the original frescoes (1378) of the great Russian (Byzantine) iconographer Theophanes the Greek. Nearby is the Znamensky Cathedral, where there are more recent (1702), but no less impressive frescoes.