Do you have little ones? A full bank account? Regular access to internet and a car?
My mom does mystery shopping, and the main reason I decided not to do it is that most all of the "shops" require you to be alone. They pay minimally, but you get lots of free meals and stuff. The catch is that you have to have the money in your bank account, and they reimburse you afterwards. For example, a few nights ago my mom went to an Irish Bar and Grill. She was required to sit at the bar for 20 minutes and order a drink. After she was done with her drink, she was to tell the server that she'd decided that she wanted to have dinner and switch to a booth. Then she had to order an appetizer, entree, and desert. The cost was about $95 (she was to bring one adult with her, so my dad went). She had to pay the 95.00 plus tip, and she will get paid $20 for doing it, so in her bank account there will be a $115 deposit in about a week. The food wasnt good, and the service was terrible.
Sometimes she goes to good places, and sometimes she buys things and gets to keep them for free, plus make a few bucks on the side. She makes about $400.00 a month doing it, and she almost always has one to do. IMO, it's about like clipping coupons. For the most part, you get a lot of crap you dont need or want and eat a lot of processed food and stuff you'd never go eat, but you get a little cash on the top. It's a fine deal if you have plenty of money in the bank, but if you have to constantly be managing your money to do it or have to get someone to watch your kids, then it's not really worth it.
I've done secret shopping. Mostly through Intellishop and Merchantile Systems, but you can look at Job Slinger to find out which companies have the best shops for you. I chose those two since Intellishop deals with the bank I already go to and Merchantile Systems does a chinese food place I like. The bank ones I can't take my kids, so that was kind of annoying getting the grandparents to watch them. I did get $16 for going in and asking questions about a loan (without filling out an application), $11 for asking a teller a question during a normal transaction, or $8 for doing a drive-thru transaction or phone call. They also had brake checks, oil changes, online school phone calls, and car dealerships in my area. It was nice and there was a month that I made over $150 just from doing these. For the chinese food it was reimbursement only, but I could take my husband and kids, which was nice. And a free meal is great, though just like PP said you have to wait for the reimbursement, which for this company was generally about 2 months.
I haven't been doing any secret shops recently because I'm in school and I just don't want another extra thing on my plate, but I'll probably do them again once the semester is over. A little extra cash is always nice!