Raise and sell livestock.
However, it is not so much about bringing money in as keeping existing money from being drained away.
Through some convoluted math using shop fees, reimbursements, credit card cash back and couponing I just turned about $400 in net expense into close to $900 of very useful material through shops.
In fact, even the livestock 'pays for itself' in the agricultural exemption on property taxes.
When my 'shop kitty' gets a little fat I send in a check to reduce my mortgage balance.
My health insurance is a 'business expense' of my mystery shopping business and is mostly deductible against my business profitability. (And face it, even if you are on Medicare your part B premium withheld from your SS check is close to $100 per month.)
We would do a fair number of the 'lifestyle' dining out experiences even if they weren't reimbursed by shops, so doing them as shops reduces personal cash flow.
As a retiree I would be bored to tears sitting at home every day, yet all of my miles on my vehicle would be personal expense without shops. As it is, only a very small percentage of miles on my vehicle are "personal miles" thanks to shops and the possibility of making stops along the way for personal errands. (There are only a few 'main drags' through here and I use them for shops with the only mileage deviation from a straight route of shops being pulling into and out of the parking lot to do personal errands.) The mileage deduction very generously allows me to protect shop income from taxation.
So for me, most of my 'business activities' these days are more to reduce normal living costs rather than looking for a paycheck. I really work on that philosophy that you make money by not having to spend it.