While Ajaccio is a great city to explore on foot, it's recommended to rent a car or take a taxi to Plage d'Argent as it is located outside of the city center. A taxi ride will cost around 30 euros and take about 20 minutes each way.
If you have more time, consider taking a boat tour to explore the nearby islands or visiting one of the many wineries located just outside of Ajaccio. If you have less time, you may want to skip the beach and spend more time exploring the city's many historic sites.