I have been a CEO of a startup 2x, so my response just reflects what I've learned is best for myself and my past companies:
* Call an existing client. Maintain the relationship. It's easy to always just focus on what's next down the pike, but the best way to get new clients is to have happy existing clients, and it's amazing how a simple call or a drink with one can make them forgive/forget the little issues that arise, or become your strongest advocate.
* Take a group of junior team members out to lunch. Again, easy to get 'head down' and rely on your exec team for insight to company morale or suggestions. But there's nothing like caring about every level of person on your team and treating them with respect and an open door policy. I have had some executives say they are uncomfortable with a CEO 'mingling' with their teams. I think that is only happening if the CEO is creating a distraction, or undermining the execs in some way (or the execs are misrepresenting or creating politics on their teams) - none of these things should happen. You should be doing more listening than talking during these outings.
* Research what's happening outside your bubble. It's easy for myopia to set in when you're working so intensively. It's good to be current and forward-thinking, and inspired by outside industries/sectors.
* Be healthy. Echo-ing other sentiment expressed before me - being physically strong helps you be mentally strong. It only takes 30minutes a day to clear your head.
* Talk to other CEOs. Whether it's YPO, old college friends, new entrepreneurs, whatever - no one understands the trials and tribulations of being a CEO other than someone who has lived it. I spent so much time focusing on what I could do for my company, I didn't think 'networking' was worthwhile. But I learned later that surviving the struggle is easier when you realize you're not alone.