St Peter Port is a compact town best explored on foot. The harbour is the natural starting point — walk along the quayside, look at the boats, and then climb the narrow lanes that rise steeply behind it toward the upper town. Everything of interest is within walking distance.
Castle Cornet is the dominant landmark, jutting into the harbour on its rocky promontory. Hauteville House, Victor Hugo's eccentric former home, is a short climb uphill and one of the most distinctive literary interiors in the British Isles. The Guernsey Museum sits within the Victorian Candie Gardens, which are worth visiting for the harbour views alone.
For something quieter, the Town Church near the waterfront is one of the oldest buildings in Guernsey, and the La Valette Underground Military Museum — built in German-occupied tunnels during World War II — is one of the best in the Channel Islands on that subject.