The first and most important items that should be packed before a planned trip to Athens are the travel documents (Identity Card, International Passport, Visa(s), etc.) as well as other documents such as airplane tickets, credit/debit cards, insurances and health insurance cards, driving licenses, etc..
To obtain Euros in advance is useful for paying for small items, gifts & food in cash.
To have a travel umbrella never hurts, especially in winter in Athens.
A pair of sunglasses, sunscreen, after-sun cream and hat & slippers should always be in hand no matter it comes to a walk around the city or sunbathing on the beach. The sun of Greece is very heavy, especially in summer months.
A trendy travel bag and purse for all necessary belongings and documents are a must. They must always be held in front because of the numerous pickpockets everywhere, especially in tourist spots and on the metro.
Ear Plugs might be of help to light sleepers because Athens is a noisy city. There are people on the streets in the early morning hours largely due to the fact that city’s nightlife starts and finishes late for locals and tourists alike.
Medical Supplies are not needed during the stay in Athens. It is always cheaper to buy local brands. However, all prescription medications for allergies and other diseases of certain brands as well as basic first-aid items are a must-bring.