Why not practise mental arithmetic, like 45 times 23. It's not really rationality, but can't hurt. It's probably good for your brain somehow.

Or you could try doing fun pointless economics or physics calculations. If you're a cashier at a supermarket, you could calculate how far the chemical potential energy in a can of soup or whatever would propel it into the air, and do the calculation for as many products as you can find. Or figure out what proportion of the money that comes through you would have had to have stolen and invested twenty years ago on order to get your current salary. Or something like that. I dunno.

(Note: Here in Australia, cashier might have a different meaning. I hope I didn't offend you by implying you were a check out guy in a supermarket.)