After these tenants move out, I'm thinking of just keeping the rental empty with furniture. It sounds stupid to give up $4,200 a month, but I really hate dealing with the homeowner association, move-in/move-out rules, and maintenance issues. Given that the condo doesn't have a mortgage and I have to pay taxes on some of the rental income, I'm not giving up that much. The condo can be a place for my sister, parents, or in-laws to crash when they want to stay in SF for longer than a week or two.
It’s a (mostly) short term, higher risk, higher reward place to invest cash that has a low correlation with the stock market, but is far more passive than buying and managing properties, has more opportunity for diversification than private placements (minimums of 5-10K, rather than 100K), and most of the equity offerings (and all of the debt offerings) provide monthly or quarterly incomes. Unlike a REIT, you can choose exactly which projects you wish to invest in.
There are three main categories of income: active income, passive income and portfolio income. Passive income has been a relatively loosely used term in recent years. Colloquially, it’s been used to define money being earned regularly with little or no effort on the part of the person receiving it. Popular types of passive income include real estate, peer-to-peer (P2P) lending and dividend stocks. Proponents of earning passive income tend to be boosters of a work-from-home and be-your-own-boss professional lifestyle. The type of earnings people usually associate with this are gains on stocks, interest, retirement pay, lottery winnings, online work and capital gains. 
Fundrise – With a minimum investment of just $500, investors of all types can make crowd-funded real estate investments through Fundrise. This means you get the benefits of being a landlord without actually having to deal with owning or managing the properties yourself. Even though we own 2 rental properties, we recently began investing in Fundrise ourselves. We love it because there is no “accredited investor” requirement, making it far more accessible for the average person than the other two options below. Follow the link above to learn more, or read our full review here.
Your income is your greatest wealth-building tool—a tool that typically requires your active participation in the form of a full-time job. You know what I’m talking about! Even if you love your job, I’m willing to bet you wouldn’t mind earning some extra income without the blood, sweat, tears, and time commitment of another job. In fact, there are several benefits. Building a passive income:
Great breakout of some common items that are (mostly) accessible to individuals. My biggest issue with p2p is the ordinary interest it generates and the ordinary tax that we have to pay. That really takes a bite out of the returns. Fortunately, I opened an IRA with one of the providers to juice the return with zero additional risk. 6-8% nominal returns over a long period of time will make me very happy. It should end up as 5-7% of the portfolio anyway, so nothing too significant.
If you're looking to get started in real estate, look at a crowd funding solution like RealtyMogul. It works similar to LendingClub - you commit as little as $5,000 towards a property. When the property is fully funded, you become an owner, and will receive your share of the earnings and appreciation in the property.  Check out RealtyMogul to learn more.