I love make up shops, I have gotten free full sized cleansers (2), free high end perfume, free high end day cream and make up. Also had a children high end specialty store shop, where I got to buy and keep the toy! I would do that every year! Anything with free groceries is good too.
My last favorite type of shops are intercept/Credit Cards shops, easy and 25% to 50% of the times the person is not even there so less work. The ironic part is I actually need a new credit card so I am shopping to see which ones are of benefit to me too. Mystery shopping has made me extremely picky but I guess it is the nature of this industry.