Mystery shopping just means you buy from certain places, seems like a lot of pizza places but they also have some retail stores and things and car shops and automotives. Some can be a huge pain in the butt and are not worth it, but I have all the free time in the world so I do some whenever I can.
Pizza shops are easiest for me since I do not drive and they require delivery. you answer about 100 questions and have to be very detail oriented but basically you evaluate the company in different ways.
I go through national shopping service... it was free to sign up. Only issue is you have to get reimbursed and it takes like a month and a half, but to me its still free food because eventually I will be getting the money back ykwim?
They also have like spencers and other shops, I just have a difficult time answering all the qns because bad memory, so the at home delivery ones i can print out the form and do it in real time and they cant see :P