I second every method you described to enhance the colors of the caps. I use the curves-layer quite often. Set to an S-curve it brightens the pic and heightens the contrast at the same time. Like magic. ;-)
Colorburn on a low opacity, combined with a screen layer has its use as well to enhance the colors.
The b/w "gradient map" (don't know if there is something like that in GIMP too) set to soft light turned out to be a good alternative to desaturation. I often found desaturating gave the pic a blurry look, so I now often use the gradient instead. It improves the contrast too.
The most important thing is to play as much as you can. No tutorial will stick with you as the things you discover yourself by simply meddling around.