I went to a fantastic workshop yesterday put on by an organization called EnvisionAction. One of the participants was talking about the fact that she is learning to recognize her periods of high energy (best productivity times) and low energy throughout the day. And she plans accordingly. She saves certain tasks that don't require a lot of thought or physical activity to do during her "low energy" times, like emptying the dishwasher.
I think this is a great reminder for us all that it can be very useful to monitor yourself and your work habits throughout the day, and to plan around your own natural highs and lows. By saving certain tasks for your down time, you can still get things done and feel a sense of accomplishment. And similarly, by tackling that big proposal or detailed project when you're at your best, you'll actually get it done faster and better. Maybe you get a bit of food coma after lunch and can't concentrate as well as you can first thing in the morning.
Another great idea is to keep a list of things you'd love to do if you ever find the time. Then, when a meeting gets canceled or you find yourself with an unscheduled block of time, you can choose something off of this list and make the best use of your found time.