Go for a stroll by the water's edge and get some fresh air at Novigrad Harbour. It's always nice watching the boats coming and going.
You'll be spoiled for sightseeing choice when in Novigrad Old Town. Novigrad Harbour is a good place to start here, but there are loads of other worthwhile sights to consider. Aquapark Istralandia, Spadici Beach, and Lanterna Beach are close to the place for ships to anchor and will fill in your day nicely.