Unlike many Old Town sections of European cities, the Castello neighborhood is up on the main hill of Cagliari (out of its seven hills), not at the port. Make sure to leave plenty of time to explore the area - from the original city walls to most of the city’s main sights, including the Elephant Tower, the Bonaria Sanctuary and the Cathedral.
You can also bring the kids up to run around in the parks, or have a picnic overlooking the city and the sea. And, speaking of which - don’t forget to check out that view! It gives you an excellent (and stunning) way to get yourself oriented with the layout of the city.