From all the tourist attractions of Istanbul The Topkapý Palace Treasury has had the biggest impression on me. This treasury contains over 5000 items of amazing Ottoman sultans collection. Gifts from other rulers of the time, clothing, gold, silver pieces, and so on, and so forth… It is a big palace so it may take you at least half a day to explore. Start your visit early to avoid crowds.

Besides The Topkapý Palace Treasury other MUST SEE attractions include:

Aya Sofya (Haghia Sofia) - Church of the Divine Wisdom, covered with mosaics and topped by a magnificent dome.
Blue Mosque (visa vie Aya Sofya) with 6 minarets; build by Sultan Ahmet.
Hippodrome (Atmeydaný) where the elections were held “Byzantium style”.
If you like shopping, you must go to The Grand Bazaar with over 4400 shops.

Important things to know:

Money
Finally new lira, with 6 zeros less, entered Turkish market. Current notes and coins will be valid until the end of the year 2005. During the transitional period both new and old currency is accepted. You can check pictures of old and new lira at NewLira.com
Taxi
Use taxis equipped with taximeters.

Shops
Do not be afraid of bargaining. It’s accepted (and expected) behaviour in Turkey.

Salesman
Especially in tourist areas you will find people who will follow you, salesman who will try to make conversation just to get you to their stores. If you are not interested just say “thank you” and continue walking your direction.

Clothing
To avoid attracting unnecessary attention, women should avoid wearing shorts or short skirts. When entering a mosque you should definitely wear long skirt and some blouse to cover your shoulders.