Can you leave an airport during a layover

I think you mean passport control/immigration. Customs pertains to any goods you'd bring into a country with you - in your case just in your carry on.

Yes - so long as you don't need a visa to enter the country you are transiting through. To save yourself some hassle, make sure you get all boarding passes when checking in.

I've done this often in different countries/airports - Madrid, Frankfurt, Munich, Singapore, Hong Kong, Istanbul, Athens, London, Warsaw - some were overnight stays and some were just visiting for a few hours.

Rule 1 Stay on topic. Make your contribution relevant to the original post.

Removed my original comments, and extend my apologies for the Rule 1 transgression

"We will be flying from Ireland home to the USA....
There is a long, overnight, layover in another country (Ireland, England, Sweden are some options).

I am puzzled.
If you are flying from Ireland to the US , why would you be going through England, Sweden or 'other' places? And why would it involve an overnight stay?"

Because they could be flying on any of a number of airlines such as for example Turkish Air, British Air, SAS, or TAP. People do not always take the non-stop, most direct route. I and my two children flew to Tokyo via Philippine Air with a stop in Manila to save thousands. Nonstop to Tokyo from SFO in business class on United was ~$9,000 per ticket but SFO to Tokyo via Manila was ~$4,000 on Philippine Air. Sure it added around 10 hours of travel time in the air on each leg (30 hours extra time all total) but we would not have done the trip for $27,000.

Yes you can, as long as you have the papers needed to go through immigration there is nothing stopping you. And if you are a US citizen a valid passport is enough for the Schengen area.

And if Sweden means Stockholm, the city centre is 20 minutes away from the airport by express train so it is to spend some time there if you have a longer layover.

You mention luggage in addition to your carry on.
You might not be able to check your luggage through to the final destination on a long layover.

When I have had an overnight layover, I had to pick up my luggage. They do not hold it for you overnight.

The last post may be dependent on the airline. I’ve never had to pick up my checked luggage unless it is a stopover (I.e. >24 hours stop) and not just a layover. Most of my travels are with star alliance and so other airlines may have different rules

In my experience, I was flying British Air with an overnight layover less than 24 hrs at LAX. Everyone had to pick up their luggage.

@travel4fun: That was probably because of US laws and has no effect on how it works in Europe.

What is your proposed itinerary and timing? That will help a lot in being able to answer your question.

Above post by aircharterbluedot reported