I would do 'anything that moves you away from the screen', and particularly 'anything that involves face-to-face interaction with another human being'. I go for things like:

• Writing (with pen) thank you cards
• making tea for officemate
• checking postroom for mail
• 'walk down corridor jobs' so that's things like checking that the boss is in, popping by another office to check what time we're going for lunch. (from a cynical perspective, this is also your 'being seen' time.)
• (added by edit) Nothing. As in a literally nothing. Close eyes, breath deeply, and do literally nothing. It's suprisingly how energising it is.

According to the Book by Francesco Cirillo, you are not supposed to do something that taxes your mind or related to work. So that's a definite "Must-not"

During this quick break, it’s not a good idea to engage in activities that call for any significant mental effort. For example, don’t start talking about work-related issues with a colleague; don’t write important emails or make imperative phone calls, etc. Doing these kinds of things would block the constructive mental integration that you need in order to feel alert and ready for the start of the next Pomodoro. You should include these activities in your Activity Inventory, and earmark specific Pomodoros to do them. Clearly, during this break you shouldn’t continue thinking about what you’ve done during the last Pomodoros. Once the break is over, set the Pomodoro to 25 minutes and continue the activity at hand until the next time it rings. Then mark another X on the To Do Today Sheet (fig. 2.5).

I had to come up with my own list of do's and don'ts for my five minute break.

Some of the things I do:

• Get a cup of coffee
• Go to the bathroom
• Grab a snack
• Glance at Twitter
• Check my blog for new comments
• Get up and stretch, but don't leave the desk area
• Stare at a wall

Some things I don't do:

• Check my RSS feeds
• Return phone calls
• Start chatting with people
• Pull out something I'd been wanting to read
• Do anything on YouTube

Those latter things, I know, will turn into 15 minute tasks. I really just want to rest my brain for five minutes so that I'm ready to dive back in when the alarm sounds. The other things can wait for a longer break.

Since I work from home and also like meditation, I take a different approach.

I only have one rule about what NOT to do: Sitting in front of my computer

Working from home means lots of time in front of my monitor and little movement, so I focus my breaks in anything that makes me move. Since I also do sitting meditation every morning and every night, I do not want those 5 minutes to go by just by being blank.

Normally I choose a physical task that can be done quickly. It can be some cleaning, doing the dishes, preparing the food I will cook later, brushing my teeth... however whatever the task at hand is, I strongly insist in clearing and focusing on the sensations of the task at hand, call it, awareness meditation. For example, while doing the dishes I do not use music, just focus on the present moment, feel the temperature of the water in my hands, the sensation of the foam...

Although it looks boring, I also found great learning QiGong. It is not the type of meditation I like doing during a long time, but it is great if you do it for those short moments because you can start/stop immediately, you move constantly and give a gentle stretch to your body.

QiGong might be one of the best 5 minutes breaks I have. Also one of the most luxury allowance for a home worker, since I could never imagine being in an office full of people, standing up every 25 minutes and do the movement dance with serious/focused while I walk across the desks of my coworkers ;-)

I don't think there are any "musts" for the 5 min break. However, the point as I see it is to do something other than work. In that regard, checking your mail is perhaps not a good idea.

Getting some fresh air, a cup of coffee or a snack, or thinking about what to do in the weekend are better options.

Since measuring my breaks, I am surprised with what you can do in only 5 minutes :-)

I'm playing Pomodorium - pomodoro technique game for 5 min.

Quite often, you have an "interruptions" list during the Pomodoro, full of little things like terms you would like to look up or quick messages you want to send out. You can do most of those terms in the short break.

But I would recommend just doing a few light stretches if you can.