Portsmouth's sights are largely maritime and military — the Historic Dockyard alone can absorb a full day with its three historic warships and several museums, and the Southsea seafront adds Southsea Castle, the D-Day Story, and a long shingle beach with views across to the Isle of Wight. Add the Spinnaker Tower for the view, Portchester Castle for layered Roman-and-medieval history, and Fort Nelson on Portsdown Hill for artillery and the panorama, and you have two very full days.
A second visit takes you to the quieter Conan Doyle collection in the Portsmouth Museum, the Royal Garrison Church and Cathedral in Old Portsmouth, and across the harbour by ferry to the Submarine Museum and Explosion at Gosport. Most visitors barely leave the dockyard — the rest of the city rewards the time.