Princess tried to get us to pre book places on their shuttle bus into town by sending a circular to our cabin (the only port they did this). The Sea Bus into town, however, is much cheaper at 2 Euros single and far more convenient landing at the pier where the trips to Delos depart.
Price of ferry to Delos is 20 Euro and admission 12 Euros. We paid 50 Euros to include a guided tour and it was well worth it because she was informative, engaging and humorous and we saw just the right amount of key sights with a brief time independently in three hours.
Be aware that the cafe on the island is very expensive with a bottle of water 3 Euros and a fresh orange juice 5 Euros.
Mykonos town is very pretty, the most attractive of all ports, with lots to explore but very expensive as well.
Read Espire's full Royal Princess cruise review