Plenty of options for renting a standard storage unit, but space must be tall as it needs to accommodate a car lift. 

Then you need to ask, do you want to be a landlord and start small with Single Family and grow slow or do you like another asset class: Multifamily (Apartments), Storage Units, Mobile Homes, Residential Assisted Living.  

Best use could be either more apartments or possibly self storage. 

If there is no cabinet over the stove, install a cheap one to hide the vent pipe and give a bit more storage.

Some other RE options include notes, syndication, NNN, REIT, a wide variety of commercial non residential (Mobile home parks (residential, but not traditional residential), industrial, storage units, retail space, office space), development, land speculation, mineral rights (My first home run was in mineral rights), etc. 

Have been presented with an opportunity for a few self storage facilities. 

I was looking at renting storage units and at a studio to rent out to students.

You don't want to start a relationship to learn about storage units and then find out they only invest in single-family homes.3.

In the back of your head, if I were you, I'd be thinking about finishing this up in such a way that it's most cost effective to "de-ADU it" if/when needed (it can be an office or a storage shed or communal rec room, no issues there).

If you have the time, I want the good and the bad. What has the investment been like for you? Easy to manage? Easy business model to run? How do you get tenants in and out?