Shawn Blanc with some good ideas:

Here are a few alternatives to what I call the “Just Checks”.

• Scroll through your Day One timeline and read a previous journal entry or browse some old photos and memories.
• Launch Day One and log how you’ve spent your time so far for the day. Doing this for a few weeks can also be super helpful for getting a perspective of where your time and energy are being spent.
• Write down 3 new ideas. These could be articles you want to write, business ideas, places you want to visit or photograph, topics you want to research, date ideas for you and your spouse, gift ideas for a friend, etc. These ideas never have to to be acted on — the point isn’t to generate a to-do list, but rather to exercise your mind and build your idea muscle. Ideation and creativity are muscles, and the more we exercise them the stronger they get.
• Send a text message to a friend or family member to tell them how awesome they are.
• Don’t get out your phone at all — do some stretches or take a 5-minute walk.

Take advantage of those moments of down time in between meetings, calls, or whenever. Allow your mind to rest for a bit or engage it by doing something active and positive.