I have to admire the restraint of the responders. Many suggestions have been provided without questioning the original premise. I can't picture any scenario that would require you to stand for an extended period of time as your presence is required but your attention is not.
Are people speaking? Can you improve your listening skills by summarizing what you are hearing in your head, coming up with the top 3 points someone made, make mental suggestions for improvements in their argument. Of lesser value, pick out the grammatical errors, identifying classes of errors that are made, count the number of ums, ahs, ers, etc.
Are you monitoring students, playground, prison yard, security check point. Can you track what is happening, focus on one individual or group of individuals, count cars, count heads, etc.?
Are you guarding property, access, on parade duty? Can you watch people, predict what they are going to do, work out patterns, build stories about them in your head, make Sherlock Holmes/Monk type observations on their dress, behavior, patterns, etc?
If you're a fly on the wall without a need to observe anything, can you design something in your head, make a list in your head, imagine what life would be like if ________, read some brain teasers ahead of time and work out the answers, do long division in your head, etc. Mentally walk through your home and identify the top 3 improvements you would make in each room if money were no object.