I would wear black trousers and a top (unless office based when I would wear a jacket. Wear something comfy (but not TOO casual iyswm). Interactive exercises? Moving boxes? Or just pointing to a can of weedkiller?
Having recruited for retail for many years I'd say the interactive exercises will be 'sell me this' or 'this is what I'm looking for what would you recommend?' And you will have to make a suggestion based on the info they've given you. I wouldn't worry too much that you'll be restocking shelves or moving stuff! Smartish trousers, top and pumps/shoes would be fine I think.