City Gate → Republic Street → St. John’s Co-Cathedral (about 1 hour, highlights include Caravaggio’s altar painting and the marble tombstone floor) → Grand Master’s Palace → Upper Barrakka Gardens to watch the noon gun salute and battery ceremony → stroll downhill through the old town in the afternoon to Lower Barrakka Gardens and the War Museum, then have dinner by the harbor. Valletta is a UNESCO World Heritage Site, and the facades of the Knights’ auberges along the main street are worth a slow, detailed look.
Take a traditional ferry across Grand Harbour to Birgu in the morning, visit the Maritime Museum and the Watch Tower for an authentic harbor perspective; in the afternoon, go south by car or bus: Blue Grotto boat trip → Hal Saflieni Hypogeum → Marsaxlokk fishing village to see the luzzu boats, and have a rabbit stew for dinner in the village.
Visit Mdina in the morning when it’s less crowded—the medieval and Baroque streets are most photogenic in the morning light, and the city walls offer views over most of the main island; in the afternoon, explore Rabat to see St. Paul’s Catacombs and a Roman villa, and stop by an old bakery for pastizzi. Return to Sliema in the evening for a walk along the seaside.
In summer, spend the morning swimming at the Blue Lagoon in Comino before heading to the airport; in winter or if time is tight, do some last-minute shopping in Valletta and visit the airport duty-free shops—all can be done within half a day.