A cruise tour will give you more options as it goes one way on the cruise- then an added interior tour. BUT round trip 7 day cruises are excellent, relaxing and scenic with the same interesting ports as a one way. Considerations for a one way- is to allow a long enough tour (also equals more money), so as to spend enough time at the destination, allow for the long transit times. You need to balance touring with your parents touring preferences. Interior touring may be better via train for time - allows more movement vs a bus but is longer. Determine their interests??? and choose carefully.