For example, bars on the first-floor windows might be very important for one tenant pool.

Two tone paint, better counter tops, new bathroom grout, clean rust stained toilets, laminate vinyl flooring that should last longer than carpet, etc. better landscaping.

And then use a plastic easy to remove cover to cover up the pipes, and then lay your floating floor on top of that?

We are putting in new flooring and doing a bathroom remodel in addition to a few minor updates.

IMO you need to start with a clean slate. 

@Hayden Wright get a bunch of market prices for replumbing a house, rewiring, new furnaces, new roof, new vinyl window cost, price per sq on flooring, price per sq for painting, etc...

The top floor unit just needs a new kitchen, some flooring and paint.

There are also bigger expenses I did like put in fans, redo floors, etc. that are longer-term improvements, but then I also did small like blinds that will probably always need to be done between vacancies...do these get categorized differently?

There was one full gut, and the rest were heavy cosmetic (cabinets, floors, paint, fixtures etc). 

I typically deliver all materials to the job site and I am still doing some of the work at the end of project like cleaning floors and windows, installing appliances, odds and ends stuff.