Take a TER train from Nice and arrive at Monaco-Monte Carlo Station in about 25–45 minutes (most trains pass through a tunnel beneath the principality, with the station right in the city center). Walk or take the elevator up to the Rock, the old town of Monaco: arrive at Place du Palais before 11:55 a.m. to watch the Changing of the Guard ceremony, then quickly tour the exterior of the Prince’s Palace, Sainte-Devote Cathedral, and the Oceanographic Museum (allow at least 1.5 hours).
Walk down the steps to Port Hercules and have lunch at a restaurant or café by the yacht marina, with meals averaging 25–40 euros per person; for a tighter budget, grab something in the Fontvieille business district or a supermarket. After lunch, stroll along the eastern edge of the harbor and enjoy views over the port from the Princess Antoinette Promenade.
Walk or take the bus up to Monte Carlo: visit the gaming room of the Monte Carlo Casino and the Opéra Garnier (daytime entry with ticket purchase), and take photos at Casino Square and the Casino Cosmopolite intersection. Then head down to Larvotto Beach for a rest, or sit quietly for 20 minutes in the Japanese Garden. On weekdays outside the F1 season, you can also walk along sections of the famous racetrack and see landmarks like the Hairpin Turn and the tunnel entrance.
Before heading back to the station, pick up some macarons and chocolates at the Monte Carlo Shopping Promenade or Rue Sainte-Dévote. Take the train back to Nice to connect with your flight or evening plans. Total walking distance for the day is around 8–10 km, so wear comfortable shoes and use sunscreen.