Castlebay is the major settlement on Barra. Ferries dock here and a short walk along the shoreline affords nice views of the neighbouring island Vatersay, as well as an outlook towards the open sea. From the post office, proceed uphill to the main road. The community shop is worth investigating. Turn right and take the next right by the old harbour. You can walk 1000 yards until the path comes to an end.
Another possibility is to walk the couple of miles from the village centre out to Tangasdale Beach. Barra is famed for its golden sands and Tangasdale is a superb example. The only feasible route is along the single-track A888 but it's quiet enough. Follow the road past Dunard Hostel and out of the village in the Vatersay direction. After a mile and a half, you will spot Loch Tangasdale to your left. This is the island's largest body of fresh water. At a roadside postbox, a signed path leads out the beach (and also serves the loch). Return to the village by the main road or head up to the Barra Beach Hotel to catch a (very limited) bus service.