Lots of things come to mind.
A nice bath & grooming session
My latest favorite is snack time. I'm trying to put weight on him so I soak some beet pulp, hay cubes and add some rice bran to it and let him delight in the mash while I sit there and read a magazine. He occassionally lift his head out of the bucket to rub some on my clothes. How thoughful!
For a more active quality time. I bring him into the ring, take off his halter, and he follows me around over ground poles, cross rails and other obstacles. I think he thinks I'm a little nuts, but he follows me at his own choice, so I guess he's not too embarrassed by his lunatic mommy.
I love spending time with him even it is soaking a hoof or wrapping a leg. I guess there really is no wasted time spent with a horse.