Casual mystery shopping?

I'm totally new to mystery shopping (haven't actually done it yet) and am trying to figure out how I can fit it in in a way that makes sense. I work a regular 9-5 job and have a family so I'm trying to find ways I can do shops on my lunch hour. I work in a very built-up commercial area with lots of national chain restaurants, retailers, etc...so it would be easy to hit a place for lunch, a cup of coffee, or to try out a new cell phone. Is this possible or is mystery shopping usually more scheduled than that? Are there companies that are more geared toward fast food/fast casual restaurants and big box retailers (that's what's around me) than others?