This is something that was quite popular in the USA in brick and mortar retail stores. There would be a couple of participants that had a short time to pack whatever they wanted from the store and run to the checkout counter to get it free of charge. It got a lot of virality, it was fun to watch and an event in itself. More than a promotion, it was rather a way to get some press and media coverage.
You could organize something like that in your brick and mortar establishment or in your e-shop, giving a couple of participants a chance for free shopping within a short time frame and record the whole user journey, whether in store or online (you can ask the participants to record their screens and then make a funny compilation).
You can create a discount coupon making certain products free and apply additional limitations on it.