Overview
Sapanca is a strange little lake town whose popularity brings together almost every kind of visitor. During our stay, Turkish and Arab families mixed with other foreign tourists; a Laz wedding was happening across the way, one hotel was conservative, and ours served alcohol. Everything coexisted in the same small area.
The lake has a pleasant atmosphere, but it is not a "wow" landscape. It works for a short break, especially from Istanbul, rather than as a destination that justifies a long detour.
I was curious why it was so popular with Arab visitors. Proximity to Istanbul and heavy Gulf-oriented real-estate marketing are commonly offered as explanations, along with property investment pushing land prices higher. I did not find an authoritative source strong enough to present that causal story as settled fact, so it remains background interpretation rather than a conclusion.
In short: good for a quick nature escape from Istanbul, but not a place I would cross the country specifically to see. 6/10.