I work on helping my mare carry her head straight, collection in gaits (including the walk!), picking up the correct lead in the canter, switching leads, bending into turns, and 'tailing', which will be important when we do Tevis 2014 (but you don't have to do that, LOL!). I also like, after 1/2 way done, to turn her back and ask her to walk a different direction than she's used to. We also have a lot of unruly, unleashed dogs, which is always a good opportunity to teach her to face a dog and walk towards it (99% of dogs will run away when challenged by a horse ... the one dog that didn't was, suffice it to say, sorta sorry and never has chased us again even though he often looks at my mare with the deepest hate). Sometimes I go off trail and slalom her through the trees to keep her nice and responsive to my legs. Sometimes I work on sending exercises even if it is for something small, like a mud puddle or creek. We stand around in water. We walk through a lot of mud. We also stop and stand there for a while doing nothing. If I feel her rushing through a downhill to uphill transition, I will ask for only a walk.
But I, too, sometimes ride around looking at leaves and sunshine. And sometimes I just space out! ;)