Monaco is about two square kilometres, which makes it walkable end to end in an afternoon: the palace and old town on the rock, the casino quarter at Monte Carlo, the harbour between them and the Oceanographic Museum built into the cliff face. Most visitors arrive as a day trip from Nice, twenty minutes away by train.
The guides here cover Monaco as a whole. The Grand Prix in late May transforms the city — grandstands go up weeks ahead, streets close and prices rise sharply, which matters whether you are coming for it or trying to avoid it. Dress codes apply at the casino, and parking is scarce enough that arriving by train is usually simpler than driving.