Yes, you can check in for your flight and leave the airport in 2026, provided you have already obtained your boarding pass and (if necessary) checked your luggage. This is a common practice for travelers with long layovers (6+ hours). However, the rules differ between domestic and international travel. For a domestic flight, you simply walk out of the terminal; for an international flight, you must officially "enter" the country, which may require a visa or an Electronic Travel Authorization (eTA) and a trip through customs and immigration. The most critical factor in 2026 is timing: you must allow enough time to clear security (and potentially immigration) again upon your return. Most major hubs like Singapore Changi or London Heathrow recommend being back at the terminal at least 2 to 3 hours before departure. If you have checked bags, they will stay at the airport; you cannot retrieve them unless you "cancel" your check-in, which is a complex process.