Located a short walk from Sultanahmet tram station is the Blue Mosque, which is immense. Get there early enough and you can walk around it's interior in your socks. Get there slightly later and be prepared to queue for the privilege. Personally I found it a bit odd to allow tourists in whilst locals are using it as a place of worship and I would have had a massive urge to see how far I could slide, which wouldn't have been well received.
It's so big you need to walk a fair way from it to fit it in.
Lines of people preparing to take off their shoes.
Beating a hasty retreat to the much nicer older building
A shot of it at night. Still pretty cool!