There are hundreds of palaces in St. Petersburg. Since you have limited time you have to choose. Catherine’s palace and Peterhof are without doubt the most elaborate of all. Peterhof has a spectacular interior, but it is the gardens and fountains that make this one unique. From port it is 40 km south west. You can take a taxi or take the hydrofoil from the Winter Palace in the city centre.
Peterhof, St. Petersburg