Am in Portsmouth now, for the second time actually. The historic dockyards are spendy to get into (29 GBP for one attraction, up to 39 GBP for an all access pass) but there are so many things to see, you probably could not do it all in a day, I am figuring probably 2 days, There are 3 Ships (Victory, Warrior, and M.33) The Mary Rose museum, the Submarine museum, and 3 or 4 other museums. There is also a WW2 museum about D-Day (Most of the invasion launched from the area), plus shops, the Gunwharf Quay area, some good pubs. If the weather is nice, there is a beach and pier at Southsea, or a trip over to the Isle of Wight.