Singapore
I love this city and would have liked to have a few days to explore and take in the sights and sounds. If you have a flight layover - the airport offers a free tour of the city, with two options. Depending on the time you have you must be in transit for this though.
As we were travelling on a NON through ticket we had to collect our luggage and re check in to BA flight back to the UK - which you can do very early allowing us time to take the easy train to the city.
We went straight to the Marina Bay Hotel to see the amazing views from the top with the swimming pool across the three towers - some 200 metre long! From there we could see the city and the 8pm light show of the Bay of Trees - I would really like to have spent more time but we needed to get aback the super clean airport for our flight home.