Many gyms have a dearth of the most popular equipment. Particularly, the elliptical trainers and similar low-impact aerobic equipment. There can often be a lengthy wait. You wouldn't want to stand in queue behind someone when you could be working different muscle groups elsewhere.
On to the idea.
Gym members are issued wristbands with RFID tags in them, which can be pressed against a panel to enqueue said person for the particular piece of exercise equipment.
A small receiver on the wristband, like a pager, will either vibrate or make an audible alert when the user's turn in the queue arises. There would be a two minute period in which one could approach the machine, log in, and begin exercising. If the user misses their two minute window, a different audible alert is issued, they are moved to the end of the queue, and the next user in line is paged.
Similarly, the concept could be applied to a bank of equipment instead of a particular unit, so if you would've chosen to enqueue on a trainer where the other user just began, it would be a bank-of-equipment-FIFO implementation.
RFID tags aren't really crucial to the idea, but it would seem like the smallest, lightest solution one could implement for short-distance wireless identification.