For true passive income that requires very little or no maintenance work, you don't have much choice.
1) stock dividends.
2) royalty from creative works such as books, music, arts.
3) advertisement from internet creative works
2 and 3 are not easy for most folks. And even these 2 tend to dwindle in time due to lack of publicity.
I exclude property (one popular "passive income" of Singapore) because it is really not passive.
There is maintenance involved. Any time an issue pops up, your tenants are going to call you up for piping, electricity, carpeting, door windows issue etc.
You probably need to get new tenants now and then and check on stuff.
I do options trading and I count that as semi-passive income too. You still have to check the portfolio every 1 week or so and renew expiring trades to get the stream of income.
Passive income is not as easy as sold in stories and books.
Passive income: Perform the action once and you'll continually earn money from it.
Photography; you can upload your photos to websites such as ShutterStock and you will be paid by number of people who used the photo.
Own a startup
Create online courses/blog and earn from advertisements
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