Go into the Kremlin and just wander the churchs. Check out the Tarhem Palace hidden behind the Assumption Cathedral. The Diamond Fund and Armory are definatly must sees too.
Directions: Tickets are sold near the Alexander gardens.
I thought it was one building but it turned out to be an entire complex full of pastel-colored buildings and museums. Very interesting.
Cast in 1586 for Tsar Fyodor I. It has never been used in action. Probably because noone could lift the cannon balls into the cannon.