Realistically assess your willingness to do washing while on your indulgent Greek vacation. If washing out underwear or a T-shirt at night is an option, you can save some weight and room. Warning: Greek islands can be humid depending on the time of year. What dries overnight at home may still be damp in the morning. And not all the latest high-performance "travel" fabrics are equal winners at drying quickly. Quick test - throw the item in the washer and check it as soon as the spin cycle ends. Does it already feel pretty dry? It's a winner. Still noticeably damp? It will be that way after an overnight hanging in your hotel bathroom.
Unless you're going mountaineering, leave your heavy boots at home.
Even if you're planning lots of walking and some hiking, most of the Greek terrain likely to be encountered by the average traveler will be best taken in a good pair of hiking shoes or walking shoes ... or even the fashion faux pas of heavy socks and hiking sandals.
For spring, summer, and fall, leave the big jackets at home beside those boots.
You'll be better off layering. Bring a thin vinyl rain jacket with a hood if you're worried about being cold; these are so airtight, you'll usually find yourself sweating.
Packed so light you're missing something crucial? Try borrowing.
If your adventures take you to the more remote Greek islands, if you're short on something, ask if there's anything you can borrow. You may be surprised what Stavros or Elena has in the back of the closet, and they'll usually be happy to share.
Remember, Greece has stores.
Find yourself short on equipment or with a "wardrobe malfunction"? Athens, Heraklion,Thessaloniki and other Greek cities have street markets with lots of low-priced market streets with stalls stocked with cheap but useful merchandise.