Late spring to early autumn (May to September) is the best window, when the mountains are at their greenest, the highland plateaus open up and Uzungöl and Sumela are easiest to reach. Even then the Black Sea coast is humid and rain can arrive any day, so pack a light waterproof; winter is mild but wet on the coast and snowy in the mountains.
Roads to Sumela, Uzungöl and the highland plateaus are winding, sometimes narrow and prone to fog. Drive slowly, use headlights in mist, and if you are not a confident mountain driver a guided tour or hired driver is the safer choice.
The region is green because it rains often, and mist can roll in quickly over the coast and mountains. Carry a light rain layer even in summer, allow extra travel time, and be ready for views to disappear into cloud at higher elevations.
The Black Sea can have strong currents and sudden drop-offs, and not all beaches are supervised. Only swim where it is clearly permitted and others are in the water, avoid rough conditions, and keep a close eye on children.