Walk along the Neva River - Kids are tired of paintings and gilded ceilings? Take a walk along the Neva River embankment.
Whichever direction you walk along the Neva River, it's a wide expanse where you can watch the boats go by, including the hydrofoil that goes to Peterhof. Across the river, you can see the Peter and Paul Fortress and the gilded spire of the St. Peter and Paul's Cathedral, once the tallest landmark in St. Petersburg.
From the Hermitage, if you go left (west) along the Admiralteyskaya Nab, there are lion statues to climb on and a nice bronze statue of "Peter the Shipbuilder," Peter the Great building a boat. Keep going along the river, until you reach the Alexander Gardens and the Bronze Horseman, a statute of Peter the Great on horseback, put up by Catherine the Great in 1778.
Summer Gardens - Alternatively, you can go right (east) along the river, to the Summer Gardens. The gardens were laid out in the time of Peter the Great and originally had mechanized fountains and a small zoo. Today, the trees and paths are somewhat untended, but it's a lovely place to stretch your legs in the afternoon or early evening. (Modest fee required to enter the gardens).