As Aiken said - testing people on drinks and/or brands vs supermarket goods is usually a good one. In particular giving them a range of gins (bombay saphire, Hendricks, gordons, tescos and low price) - then ask them to pick which one is which. Surprising how wrong people get them. I seem to remember they did a blind taste test on some cookery show with an actual wine taster. And out of a selection of reds he totally rubbished a bottle of Chateau Petrus which costed €3,000 and lauded Morrisons finest. So it produces interesting results.
I also think back to when i used to work in a posh hotel. And we served afternoon tea. There were scones with the afternoon tea. And the pastry chef was off one day and we didnt have any. So we went down to Tescos, bought a pack of their average scones. Stuck em in the microwave for 15 seconds, and served them up. We also didnt have any jam, so bought Tescos seedless raspberry. No one noticed, complained, or said anything. And we still charged them the extortionate price for them. We also served people sparkling white wine at their champagne reception on New Years eve. Out of a hall of about 500, no one noticed. Its amazing how blind people become when you present things well. Even though the underlying product is pretty much garbage.