Disclaimers: I am not a professional plumber. I am not affiliated with any companies mentioned.
How old is the property? If it dates from before the late 80s or so, and the bathrooms haven't been upgraded, you might be rocking 5-gallon-per-flush toilets. The next step was 3.2-gallon, then 1.6-gallon, and now 1.0-gallon. Somewhere around the 3.2-gallon-per-flush era, it also became a requirement that the flush size was marked on the toilet - usually it's right behind the seat bolt holes, where you can see it by looking down at the toilet with the lid closed, but it might be stamped inside the tank. If it's not marked at all, measure the tank and bust out some geometry to figure it out.
If you still have 3.2 or 5.0 gallon toilets, you might consider swapping out for 1.6 gallon ones. I have had two American Standard 1.6 gallon toilets in my residence for 10+ years now, and they seem to work OK. It probably helps to buy a notch or two up from the $99 toilet that the big-box store has a million of. You might consider one of the soft-close lids to help sell it as an upgrade. If any of your tenants are, uh, mature and experienced, and you think they might stay there for a while, you might also consider a toilet with a higher seat height - they're a little easier for people to get up from.
I personally wouldn't install a 1.0 gallon toilet - they come with a warning that you can't install them where that toilet is the only thing happening (think a small commercial storefront or similar), because they need the extra water flow from a tub/shower drain to actually move all the poop through the pipes.
If the toilets are all recent enough to be 1.6 gallon, how about the valves inside them? If the tenants don't pay the water bill, they might not tell you when the toilet runs all the time. New flush valves (the flapper in the bottom) are cheap, and most big-box stores have a pack of 4 or 6 of them for a little bit of a discount. The Fluidmaster ones have worked the best for me - and when one didn't work after a year or so, they actually made good on their warranty.
If they still have ballcock fill valves, replace them with Fluidmaster 400A fill valves for less maintenance hassle. Replace the rubber valve washer in them every 5 or 10 years or so and that's it. I have also seen, but not used, replacement fill valves that give you two sizes of flush - a couple of quarts for light work, or the whole tank for bigger jobs. I've looked at them in the store and concluded they had a lot of moving parts, so I'm not sure how much I would trust one in a rental.
Shower heads are also available in lower-flow versions. Since some time between 1990 and 2010, they are also required to be marked with their flow rate, so you might be able to tell what you have now. On mine, from 2009, it's molded into the black plastic of the shower head, so you need a good light to see it. If you can't find a marking, and you really want to know, use a bucket and a stopwatch.
A couple of things to look out for: some of the low-flow heads don't give you a very strong spray. You might buy one and try it at your residence before buying a bunch for your rental units. Mine is a Delta, 2.5 gallons per minute, and it does reasonably well. Also, sometimes the low-flow part is implemented by a small nozzle/restrictor that goes into the back of the shower head as a separate piece; a few tenants might be clever enough to unscrew the shower head, take that part out, and screw the head back on.
If they don't already have these, you might "sell" this as an upgrade by installing the kind that have a flexible hose (so you can hold the shower head in your hand) and a bar that the shower head can be hung on at different heights. These require a little more maintenance than a fixed shower head, though - eventually the hose leaks, and sometimes little kids might try to swing from the loop of the hose. Or, get the fixed kind that are like 4"-6" diameter, like some hotels have.
Is there an outside hose bibb? There probably has to be at least one, per building code. Maybe the tenants like putting up a sprinkler or filling up a kiddie pool for their kids (or pets?) in the summer, so you might not want to shut that down. If those things don't happen, you might consider installing the kind of valve that takes a four-sided key to open. Again, some tenants might figure out how to fake it with a pair of pliers, or go buy the key at the hardware store themselves, but it's an option.