I'm in the process of planning this now. On the September 3rd (now "Celebration") cruise from Portsmouth. So planned a trip around it. Flying FLL-LGA (JetBlue), EWR-ATH (Emirates), ATH-JTR (Aegean), JTR-LHR (British), Overnight in London, Train Waterloo-Portsmouth. Then after cruise, train Portsmouth-Gatwick (Have to make a 4pm flight that day!), LGW-OPO (British), train Porto-Lisbon, LIS-MIA (TAP).
Will be 2 days in Athens, 3 days in Santorini, 2 days in Porto, 1 day in Lisbon. I looked at hotels in Portsmouth and Southampton, and didn't see anything that was appealing to me.