The size, type and weight of your luggage is important. Over the years, some students have invested in a new piece of luggage for this trip, and they have found it useful year after year. If you purchase your luggage at REI with a membership, you can return it at anytime (even after 50 years!).
Many students recommend that you bring a backpack style bag rather than a rolling bag. In certain places, like Hydra, it's better to have something you can carry on your back. It's also important that you don't bring too many bags. For this reason, a bag that has a detachable daypack is an excellent choice.
In general, I endorse REI for purchasing your luggage. They have great options, knowledgeable salespeople, and an amazing return policy. If you're thinking about buying new luggage, visit an REI.
Here are some options:
The REI Grand Tour Travel pack is an excellent option. It's the right size and comes with a detachable day pack. This way, when you're headed to the airport or walking through Athens, or to the train station, you can have everything on your back with your hands free to buy tickets.
The Eagle Creek Thrive Vita Travel Pack is another good option with similar features to the Grand Tour Travel pack. Again, it has a detachable day pack and allows you to carry your stuff on your back.
The REI Stratocruiser Wheeled pack is my choice because it is a backpack and a wheeled bag. So, under most circumstances you can wheel it around, but when in Hydra or walking over cobblestones, you can put it on your back. Like the other bags, it has a detachable day pack. They day pack of this model also acts as a camel pack, meaning that you can use it for biking and other sports.
The REI Vagabond Travel Pack is another good option. This one does not have a detachable pack, but it's easy to use when you fly.
A roller bag is a good option, too. The majority of students use these bags with no problems. With these, you have two basic options. Medium size and large. I would suggest that you bring a medium-sized bag (21 inches) and not a large-sized bag (25 inches or larger).