Depends a little on your situation. If you have additional investment money after 401k and IRAs, you could invest in non-retirement accounts that provide dividends. I throw some extra each year into more of those. It's fairly easy to liquidate in case of need, although there is always the risk of market downturn. The other method I have is rental property. I'm fairly handy, so I can do easy to moderate repairs myself, and it's in a college market so I always get grad students to rent it out. That provides a lot of tax benefits, and I can also visit family when I go back to check on my property, so travel can be written off as a business expense. Rental property will probably be what I expand as my wife transitions from resident to full-time physician and we can project budgets a lot more stably in the future. If you don't have the time to actively manage a property, you could get a mgmt company to do it for you, although they usually charge around 10 of the rental income and it changes your tax structure a little if you are an active or passive manager/owner.
Oh, that kind of passive income. That's exactly my strategy. Just bought an office building for my own office space, and renting out the other offices. Makes my own offices overhead free (income from the other offices covers it and the mortgage) and I can customize my layout without worrying about a landlord raising my rent. I'm lucky that my husband has some real estate management experience and is willing to do the bulk of the work in his spare time.
I'm trying to figure out how to generate passive income from my current business. I've been writing a book, on and off, for the last five years, and if I could figure out how to free up some time from direct earning, I could finish the thing. I've come to realize it will never be done unless I set aside time for it (as in, block out time in my appointment calendar the way I would for a patient) but there's that need to earn my salary!