I had an evening flight from Dalaman to Istanbul so, rather than fly straight out the next morning I elected to stay a day and explore.
I stayed at the Side (pronounce 'Siddy') hotel which was extremely reasonable at €35 per night including a turkish breakfast. The hotel is situated near the Blue Mosque within easy walking distance of all the sights.
One could spend weeks exploring Istanbul, but in one day of brisk walking I saw The Aya Sofia, The Blue Mosque, The Cistern, The Grand Bazaar and I even had a turkish bath. I did this because the sign said it was one of 1000 things you must do before you die. There was nothing weird or kinky about it if you don't mind a burly, moustached Turk in a tablecloth apron throwing you around on a slab of marble and scrubbing you (almost all over) with a rough mitten.