On page optimization (essentially rewriting all your content so it’s unique and will help rankings), off-page (all your local listings, directories, etc), back-link building, consistent content creation, getting that content on high authority pages as a third party contributor, ideally linking back to your site, etc.

I would not even consider what you are suggesting until after he has explored the HELOC option.

Thomas S. has pointed out California investors enjoy some unique tax benefits... knowledge is the key.Just my personal opinion: ALL investments without knowledge is speculation...

There are also some unique toilets for confined spaces.

I may have to explore the Pro membership and spend some time on the Marketplace page where I've been informed I can advertise with the paid membership.

Also, I have been exploring online lead gen. more and more recently.

In the meantime, I'm renting an apartment, and my lease is up in June.There are a couple of blue collar suburbs of Cleveland that I've been exploring and would be perfectly happy to live in.

Unique Amenities that help in bookings?

Some quick searches seem to indicate that the market in Oahu is somewhat unique (rules limiting short term rentals, lots of leaseholds, etc.).  

Everyone's personal preference is unique but as Thomas S. mentioned, I would never pay off a mortgage.