Buy a ticket and join the crowds at a game at Louis II Stadium. If there's nothing scheduled while you're in town, you might like to check if the stadium offers a tour of the grounds.
You'll be spoiled for sightseeing choice when staying in Fontvieille. Louis II Stadium is an ideal place to begin here, but there are many other fine sights to discover. Casino Monte Carlo, Villa Ephrussi, and Prince's Palace of Monaco are near the sporting facility and and will cap off your day nicely.