Some others that I've used/am considering/had recommended to me:On-site, coin-operated laundry machines (most relevant for multi-family properties)Renting out storage space in basement or on-site shedRenting out unused off-street parking (where I live, winter RV parking is at a premium)Converting non-conforming to conforming rooms (including basements) to allow for re-organization and classification as bedroomsAdding accessory dwelling unit(s) to your property (may not even require separating lots)Separating utility metering (or installing individual unit readers) on multi-unit properties that are currently metered togetherDepending on your market, furnishing a portion of your units and renting them out fully furnished to shorter-term renters*Some (most) of these will depend on how individual properties are zoned, what zoning requirements are in your locale, and what there is demand for in your market.

Computer, printer, walkie-talkies to take the parks (not everyone wants to use their phone when abroad because of the cost), efficient ac units, large washing machine and dryer (will save you money on electricity and water).  

Just recently purchased a hand writing machine. 

I have stayed away from yellow letters and machine produced handwriting.

TAMPOON MACHINES, someone out there today is sitting back counting stacks of cash as the Tampon Machine King/Queen, does that mean it's viable for the rest of us?

As Chris has pointed out they tend to have higher overheads as they are such a big machine so you may pay more for their property over a smaller outfit like Curts.

A few good tools that people use for this are Propstream, Deal Machine, building relationships with attorneys/wholesalers/other investors.

What is the cost to replace galvanized pipes for just a one bathroom house, a kitchen drain pipe and a washing machine drain pipe?

Build the machine in your backyard, and after it’s built, then send it out to other markets.

Even some nice earplugs or white noise machine could be a nice touch (I think the neighbor has dogs from what I was told).