The Greek Islands trick is a travel hack that can result in big savings on airfare. The “trick” is that if you can get a great deal on a long-haul international flight to the continent/region you want to visit, you can pair that with a cheap regional flight (or even a train or bus ride) to get to your final destination for significantly less than it would’ve cost to book the whole trip on one itinerary.
There’s a major benefit to using this travel hack, aside from the obvious perk of saving money. Namely, it gives you flexibility. Since you start by booking a cheap flight to the continent you want to visit, you then have the time to decide where you want to go; you don’t have to book the second, cheaper regional flight (or bus/train; check Rome2Rio to see what's best) right away.
So, if there’s an amazing deal to Brussels but you’re not super excited about Belgium, you can snag the deal and figure out later where you actually want to go—which also means you have time before you have to pay for that second flight.
Additionally, if you do want to check out the entry city you can build in a layover. You could land in Brussels and immediately depart for, say, Budapest, but then at the end of your trip, you could return to Brussels for a long layover to check out the city.
This multi-step approach does require a few additional considerations. First, make sure you know exactly what you’re getting from the cheap long-haul flight that gets you to the continent; if it lands at a less-trafficked airport, you might eat up any potential savings transferring to a different airport to catch your second flight. You should also check out the cost of that second flight to make sure it’s going to be as cheap as you expect.
Remember, as far as the airline is concerned, you’re flying to your first destination, full stop. Even if you only plan to stay in that country long enough to catch your next flight out, you will still need to follow all that country’s rules regarding visas, customs, and immigration. You’ll need to go through immigration and customs (and collect any checked bags from the first airline and then recheck them with the second airline) and then go back through security.
Also remember that because your two flights aren’t connected on the same itinerary, if your first flight is delayed and you miss the second, the second airline has no responsibility to make accommodations for you; you may be stuck buying another ticket. Because of the inherent risk in booking two separate tickets, we recommend allowing plenty of time to make your connection, though just how long will depend on factors like the airport, whether or not you’re checking bags, time of year, and your tolerance for risk.
If, for example, you’ve landed in Brussels and are catching a flight to Budapest and you know there are several more flights after your scheduled departure with an average price of $50, you might feel comfortable leaving a little less padding in your connection time. If you do miss the second flight, you can hop on a later one and won’t be out too much money.
On the other hand, if you’re flying back to Brussels from Budapest to catch your flight back home, and it’s the only flight offered by that airline that day and a replacement flight would cost upwards of $500….well, you may want to consider getting back to Brussels the day before your flight home.