I agree that shoes are half the game. If you go somewhere like the Parthenon, the surfaces are uneven, there can be awkward steps and the ground can be very slippery. If you have mobility issues, it can be a challenge and tiring. However, in my view, it would not be overly strenuous to most people, especially if the weather is nice. However, the cruise lines are always going to be very conservative when it comes to describing the risks associated with an excursion.