A few years ago, I worked a short stint (2 months) on strike duty at a manufacturing plant. 12-hour shifts of loading various sized chunks of metal on the front end of a furnace conveyor. Sometimes it was moving fast and I could barely keep up, and other times I could practically take naps in between conveyor moves. Here are some things I did to pass the slow times. Like you, computer stuff not an option.
1. Cleaned. I obtained some degreaser and some rags and wiped down equipment and piping. Found a couple small leaks in the gas line as a result. :-) And gave me a sense of accomplishment.
2. Exercised. We had buckets of aluminum pucks used for spacers, and I'd use them to do squats, tricep presses, etc.
3. Did mental exercises. I had a clipboard where I tracked multiple productivity measures. Times for each row of metal chunks I put on, times for the crew downstream to do die changes. I'd estimate how far down the production schedule we'd get, based on the product mix.
Now, I understand that manufacturing is not retail. But you might be able to adapt some of these categories to a situation where a customer could walk in at any time. What kind of retail operation is this?
Cleaning: If it's clothing, you can go through the racks and make sure everything's organized in the right sizes. If it's cars, you can polish the water spots off the windows.
Excercise: If it's something where you can see customers coming before they enter the store (i.e., strip mall vs. indoor mall), then you could sneak in some squats or yoga balance poses.
Mental exercise: Track the rate that customers come in the store and graph the ebbs and flows. Keep track of history and attempt to predict how many people will come in during the next 30 minutes. Track them by gender or height or weight to add factors to make it interesting.
Have fun. :-)