Valletta is a walkable city, and many of the attractions are within close proximity to each other. However, consider taking a bus or taxi to the Blue Grotto and Hal Saflieni Hypogeum. The bus costs €2 and takes approximately 45 minutes, while a taxi costs around €30 and takes approximately 20 minutes. Alternatively, consider joining a guided tour which includes transportation.
If you have more time, consider visiting the Lascaris War Rooms, a historic underground complex. For foodies, try local dishes such as pastizzi and rabbit stew. If you have less time, skip the Three Cities and St. George's Beach. Don't forget to wear comfortable walking shoes and carry a bottle of water, especially during the hot summer months.