catch the train to the next port the next day. in country. we figured it'd be no big deal. we wanted to see the city at night, that's the problem with cruises is that you miss the stuff at night at the ports. anyway...the cruise line freaked out when we told them our plan. if we purposefully didn't get back on the ship at a port we would not be let back on at the next port. unless there was a very explainable reason, like your medical situation. but i guess we could have lied and said we got lost or something.
anyway, i thought it was interesting. they won't let you get off and get on at the next port on purpose.