Duplicate your picture layer by dragging the layer to the ‘new’ icon in the layers palette (ctrl+j).
Apply a gaussian blur (Filter > Blur > Gaussian Blur…). Blur it enough that the detail disappears but the shapes mostly keep their form.
In the layers palette, change the blending mode from ‘Normal’ to ‘Overlay.’
If you look at the before and after, you can see that this method makes the light tones lighter and the dark tones darker while softening it a touch. Basically, it softly boosts the contrast. If you want a more dramatic effect, try changing the blending mode to ‘Vivid Light’ instead of ‘Overlay.’
Try it on all kinds of shots: portraits, nature shots, you name it. I use this method ALL the time. It works so well with everything!
Note:This is for photoshop, I dunno if it works for GIMP too.