Monaco is shaped like a U, but a U which is high up in the air ! The attractions are neatly, if irritatingly, divided between the two sides. The palace (boring inside, worthwhile views from terraces, Ruritanian changing of the guard) and the old town (typical Mediterranean) are on one side, as are the 'Jardins exotiques' (a garden full of cacti etc; steep paths, when you've seen one cactus...) with its underground limestone caves full of stalactites and stalagmites (worthwhile),the cathedral (dull, only rated by locals!), and a good, interesting, aquarium (in the Oceanographic Museum buildings; avoid the rest of the museum as boring). The stadium, the Stade Louis, is on land reclaimed from the sea and is at the foot of the rock on this side of Monaco.
The rock on the other side of the U has the Casino square. The square has the Casino (carry your passport to enter the Casino) on the seaward side (fine views from the terraces behind it) and the Opera House next to it,and the Hotel de Paris (biggest display of flowers you'll ever see in one vase inside its foyer; look out for the marble turtles on the ceiling; biggest display of exotic, wildly expensive, cars you'll ever see, outside it ). An enormous pavement cafe is on the third side (popular with locals and visitors alike, not overpriced).
In the middle of the U is the harbour.The road from one side of the U to the other sweeps down like a roller coaster, along the harbour edge and up again. There are few big yachts (they're in Antibes , down the coast), but none of them are exactly small.
The area around the stadium will be shut to traffic for some hours before kick off and the streets nearby congested. If you are on the opposite side of the U, getting to it may be difficult, though no doubt cab drivers in the casino square know a way. That will probably involve dropping you up near the station or somewhere, up the hill. Everywhere in Monaco has outside lifts (ascenseurs) and escalators, to get you up and down the rocky terrain, so being up the hill is no problem; just follow the crowd.
To get around use buses. There is a ticket which buys lots of journeys, and buses are frequent. Cabs are numerous too but tend to be found in numbers only at Casino square with some near the Palace.
Le Petit Train, a tiny 'train' consisting of tiny open-sided carriages drawn by a mock locomotive, does a whole tour of Monaco all day and is an easy way to take it in.