Then showed up from a 16 hour work day with a sink full of dishes, all my beer gone, uncleaned up cat vomit in a planter, excuses on why there isn't rent and "Oh by the way can you grab a pizza, I'm a little short?"

Just to give an update....The weekend after your feedback we got to work to do some of the minor changes to the house, repainting the universally hated diarrhea colored front door lol and adding a flower planter.

Next step is painting the planter black and new driveway (driveway is all broken up on both sides!).

Try to do what you can to make your sidewalk and parkway attractive too - plants, planter boxes, etc. 

Tell them smoking outside is just fine (with an adequate ash can - planter with dirt is great), but you are not going to allow them to smoke in the unit, period.

You could put out some bright colored planters and some beautiful taller plants in those that will dress it up.I'd maybe do some interior lighting around the windows to give it a pop if that's what your looking for, I work in a old traditional building,, we did LED track lighting that's in the plastic tubing around the main floor windows and really pops when you drive by. 

Nothing to fill in with an above ground pool, unless you want a very large raised planter :D  .

I work seasonally as a tree planter and have done that for the last 12 years.

We did make a few upgrades like painting all walls and cabinets, cleaning up all the trash outside (there was quite a bit), de-mossing the roof, and some landscaping improvements like putting in a path and adding planters.

Depending on the size, you could house animals, if your city doesn't have a community garden you could create one and have people pay a small fee either up front or per month for a planter bed, like @Jill F. said maybe storage units or garages.