If you have a multi-unit property, check into coin operated laundry equipment. Depending on your locale, you can probably find a company that will provide them, maintain them, even give you a cash " decorating" allowance to set up the laundry area, and still split the income 50/50. Of course you pay for the utilities to operate, but the income adds up.
Same goes for other vending machines, especially soda, and if you have sufficient traffic (for example an on-site office) a candy/snack machine too. Everyone has change to drop in, and the kids are right there pestering mom and dad to cough it up! Again, YMMV, but I have had name brand distributors put in a machine at no cost, I set the sales price, they stock it, and send a check every month. Sweet (pun intended).
I sometimes let a tenant paint interior rooms on lower end properties. I reimburse them for the paint after inspecting to see the job was done reasonably well. I advise them not to get paint on the floor or woodwork, and limit their paint choices to pastels so they can be easily covered if need be, but usually they turn out good enough to just re-rent as is once the current tenant moves out. The current tenant now has some " sweat equity" and typically is an excellent tenant, and I save the high cost of a contractor paint job (or the time and effort of doing it myself) on a property where appearance is not a priority.
If you are providing the utilities, meter ac's or other high use circuits, then bill the tenant for excess over a predetermined average cost which has been included in the lease document. Along those lines, put electric water heaters on a timer so they are off several hours a day when not normally needed.
Probably the single best thing you can do to increase cash flow over the life of your investment, is perform simple, routine, preventative maintenance and inspections. Be proactive not reactive. Run your business, do not let the business run YOU.