If you come to Istanbul, there is a wonderful place to see. Atatürk Arboretumu.
Actually, the best time to visit is spring time. It will be very nice in the summer, of course! But be sure to have cold water near you, because there is an area where you can walk for hours :)
Sit on the benches near the lake and close your eyes, you will hear the sound of birds. This is the peace of mind I do not change anything. Everywhere looks lush and perfect. Do not forget to take photos when you come here :))