Late spring (May to June) and early autumn (September to October) are the sweet spots, with warm sea, green hills and thinner crowds than midsummer. July and August are hot, busy and pricey, with packed beaches and higher room rates, while winter is mild but wet, and Corfu is famously the greenest Greek island precisely because it rains more; many resorts and some ferries wind down out of season.
Summer sun is strong, especially at open sites like the fortresses, the Achilleion gardens and the beaches. Carry water, use sun protection and a hat, and plan the hottest early-afternoon hours around a shaded lunch or an indoor museum.
Corfu's water is beautiful but some coves, especially around Paleokastritsa and the west coast, can have sudden depth, boat traffic and afternoon wind or currents. Swim in view of others, watch children closely, and take care around rocks and jet skis.
Many of the island's best spots are reached by narrow, winding and sometimes steep coastal roads. If you rent a scooter, ATV or car, drive defensively, wear a helmet, avoid riding at night on unlit stretches, and do not attempt it if you are inexperienced.