A salt lake is what happens when water flows into a basin and only leaves by evaporating. Everything dissolved in it stays, and over thousands of years the water becomes so dense that a human body cannot sink in it — the Dead Sea is about 34% salt against the ocean's 3.5%, which is why people float there reading a newspaper rather than swimming.
The same chemistry produces the other thing salt lakes are famous for. When a thin film of water sits on a perfectly flat salt crust, it becomes a mirror: no waves, no depth, no horizon. Salar de Uyuni is the largest and flattest surface on Earth — 10,500 km² varying by less than a metre — and after rain the sky lands on it completely. Satellite teams use it to calibrate instruments for exactly that reason.
So one heading covers two very different days out, and which one you get is a question of season and lake. Some are floats, some are mirrors, a few are both at different times of year. Three practical notes: salt water finds every cut you have, it destroys camera gear and shoes, and one lake on this list — Natron — is caustic enough that you do not go in at all.