The old town is tiny — you can walk anywhere inside the walls in five minutes, and to either beach in ten. The streets are steep and largely cobbled or pedestrianised, so a car is no use once you're in. Park at the top of town and walk down.
Local buses run along the coast to Saundersfoot, Manorbier, Pembroke and beyond. The Coastal Cruiser bus links into the wider Pembrokeshire Coast Path bus network in summer — useful if you want to walk a stretch of the path and take the bus back. Bike hire is available from a couple of shops in town.