Cherbourg is easily accessible from Normandy and the rest of France and is around 4 h from Paris and around 7 h from Bordeaux. There is not much traffic except during rush hours. There is limited parking close to the old town and you may have to pay for it during the day.
Cherbourg can be reached from Paris St Lazare on a direct train, otherwise there are services to Brittany and to (sometimes) the southwest of France. Gare de Cherbourg is about 5 minutes' walk from the old town. It is also close to some hotels and a Carrefour but it is difficult to find food near the station.
La Cité de la Mer (link) is a maritime museum situated in the restored trans-Atlantic railway terminal in Cherbourg. Visitors can explore an aquarium, an ocean diving area, an exhibit about the Titanic and a French nuclear submarine. The monument was built in 1933 and the museum was opened in 2002. It is a 10 minute walk from the old town and costs €18 for adults.
Château des Ravalet (link) is in Tourlaville (you will need a car or taxi). It is a picturesque château set in nice parklands and is a popular place for picnics. The interior of the château itself is only opened occasionally, however, the gardens are open daily and are free to access.
Phare de Gatteville is located at the north-east point of the Cotentin, about 20 minutes drive from Cherbourg. It is a lovely, peaceful spot, surrounded by fields and villages. For a few euro, you can climb the 349 steps for some incredible views.