Income from the performance of services directly related to the use of a vessel or aircraft is treated as derived entirely from sources in the United States if the use begins and ends in the United States. This income is subject to nonresident alien withholding if it is not effectively connected with a U.S. trade or business. If the use of a vessel or aircraft either begins or ends in the United States, refer to Transportation Income in Publication 515, Withholding of Tax on Nonresident Aliens and Foreign Entities.
4. Save, build and run a bread-n-breakfast place. Look at airbnb Vacation Rentals, Homes, Apartments & Rooms for Rent - Airbnb for inspiration. My wife runs one (Firdaus, Naukuchiatal) , it is not an income as of now but if you are on it for enough time, it would be, when you grow old. It doesn't have to be a fancy and glamorous thing. It could be a  2 nice-n-clean room in a city where you live. If you have a big house, it could be a  part of your house.

For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.