Istanbul is a really great trip for 2 or 3 nights. Tons of history, souks, fantastic restaurants, beautiful views over the Bosphorus, great nightlife, Turkish baths, etc. I usually stay in the Sultanahmet district as it's walking distance to the main attractions such as the Hagia Sophia, Blue Mosque, Topkapi Palace, Grand Bazaar, and Spice Market. There are a number of good hotels in various price ranges near the Four Seasons Hotel.
The rooftop bar on the Four Seasons is great. Also there are a number of rooftop bar with excellent views located near the Swisshotel Bosphorus - Vogue Bar and 16 Roof are two good ones. And the Kempenski Cirgan Palace and the Four Seasons Bosphorus have great restaurants and outside patio bars right on the water…very picturesque.
One suggestion is to go to the Pera Palace hotel for a drink and then have a dinner reservation at the Michelin star restaurant Mikla next door. Lastly the Hurrem Sultan Hammam next to Hagia Sophia is a can't miss Turkish Bath…definitely worth doing.