Some greens take on a bitter taste when they are old (eating kale in July will make you think it's disgusting).
There are some other greens that are generally less mild tasting, too. Swiss chard, for example, was one that I never really cared for. I'm a pretty big fan of kale and spinach, though.
Good eats! I need to work up a camping trip with the family this fall. We haven't been having enough adventures lately...