You have several options and have to pick according to your situation.
IF you are in a brutal situation with mega-hours and huge stress, then go into survival mode. Concentrate on keeping your body, mind, and soul sane. Get as much sleep as you can (top priority). Try to eat well (at least, avoid fast food). Get whatever exercise is easiest for you, even if it's a fast walk around the parking lot a couple of times while on the phone. If you can't meditate, sit quietly for 10 minutes and just be still. Enlist your Spouse, if you have one, and ask outright for pampering, swearing to make it up to Spouse after retirement.
If your situation is less dire, then you can think beyond the immediate day. Make LOTS of appointments - get your medical in before retirement, see retirement couselors in various units, meet with colleagues. But try to stay away from useless meetings! Say that you are concentrating on getting things finished up. Check firecalc daily. Do at least one countdown clock, possibly two. And think about your first week off - if you are the planning sort, think about your feast, your first night's sleep, your lunch out with friends. Enlist Spouse to plan something fun. If you plan to travel, prepare however is fun for you to prepare. Lists were fun for me.
If you are powering down on just one or two days a week, well, you did it right already!