Sultan Ahmet

Wear “modest” clothing! Shorts are not permitted, men and women alike will be expected to cover up (robes are provided if you’re inappropriately dressed). Women are not required to cover their hair, but it is appreciated. Shoes are not permitted; you’ll be given a bag to carry them in. Remember: this is a functioning house of worship, not a museum. Act with respect, and respect will be returned. Donations are collected as visitors are leaving; not required, but a good thing to do. It’s frequently crowded, because visitors are not allowed when worship is going on, so the times you can see it are limited. It’s a beautiful building inside and out, definitely worth seeing!

It’s beautiful, but if you have to choose go to Aya Sofya instead.
I wish Istanbul could get rid of those pesky touts hanging around outside trying to sell you everything from rugs to their sister.