I never use bug spray, but this summer's been so wet that the insect population has exploded. The towpath has been especially bad and I actually broke down and bought bug wipes at the bike shop in Hancock when I was riding up there two weeks ago. So +100 on the bug repellent recommendations.
For the same reason, +1 on the recommendation for sunglasses with interchangeable lenses. You'll be glad you have clear lenses if you ride at dusk. The gnat clouds get pretty big. And try not to ride with your mouth open unless you want an unexpected snack
Last, the number one rule of C&O riding, especially for a summer like this one:
Never run over a stick unless you are positive that it is a stick.