Rental. Having bought an apartment abroad, it can be rented out. This is the best option for passive income. Having once invested in real estate, you can make a profit for many years. But, again, you should carefully approach the issue of choosing a country. For example, in Germany or the Czech Republic it can be rented all year round and evenly profit. As for Turkey or Bulgaria, in the holiday season the cost of renting housing will be much higher than in the same Czech Republic. But, in the remainder of the year, finding customers will be very problematic.