Since you'll be visiting several countries on a Mediterranean cruise, the first thing you'll need to pack is your passport. Make sure it's up to date. If you need any visas or other identification, pack those as well. Pack your travel information and your insurance cards.
Dressing in September can be tricky---the weather could be hot or cold. Packing clothes to wear in layers is always a good idea. Include a jacket and a lightweight raincoat. Check with your cruise line to see about formal dining, as many cruise lines have formal dining nights when tuxedos and evening gowns are required. Even if you don't need formalwear you'll most likely want something dressy for a special evening onboard.
There are several things that you'll also want to bring with you. Sunscreen and a hat are important---the sun can still do damage to your skin in September, and no one wants to have a miserable sunburn on a vacation. Sunglasses are also a good idea. Don't forget a camera and extra film or memory cards. You might want to bring maps and guidebooks for shore excursions so that you'll have an idea of what you want to visit. A phrase book for the languages that you'll be encountering is also a good idea.
Don't forget any medication that you might need. Medications need to be in their original containers, don't throw them into a pill box. It's also a good idea to bring a record of immunizations with you since different countries have different requirements. It's better to be safe than sorry.
Pack comfortable clothes for daytime and shore excursions, and don't forget comfortable walking shoes. You'll want a modest bathing suit for the ship's pool along with a cover-up. Don't forget water shoes or flip flops for walking to and from the pool.
