More specifically, I am looking for investment vehicles other than "playing the stocks" in which to generate long-term wealth and an additional supplement to retirement.

It isn't my best lead source by far, but it can be a good supplement.

The cynic in me says that REI is not giving them enough cash flow and they need something else to supplement it...

I'm looking toward retiring from my 9 to 5 job in about 10 years, and want to supplement my 401(k)/IRA and Social Security (if it's still around) income. 

My wife and I started out buying a two-unit apartment house about 7 years ago with a similar goal... buying 1-2 small multifamilies per year to hold and pay off the mortgages to cash flow supplemental income.

Also to retire he needs the income from the rental to supplement his pension.

I will get as many as six, and use the best 5. 2 supplement the best 3.

I’m planning on purchasing a few OOS properties to supplement Ca properties.

You can use cash generated from the losses if any to supplement your 1031 purchase.  

My older brother, who is currently flipping his first home in Philadelphia, enlightened me on the possibility of real estate as not only a supplemental income but more so a way out of my house via house hacking.