The Levels (Color>Levels) tool in GIMP has a button/option referred to as “Auto Input Levels”. This simple to use (single button selection) tool can produce rather remarkable results and even when it makes very little change it is quite likely to be positive.
Unfortunately, I haven’t been able to find anything that explains what it is doing in any detail. Given how positive the results tend to be it would be very instructive to know what it actually did to any specific image on which it was used. A related question that comes to mind is can the same thing/s be done using the various (manually activated) controls?