Charminar: One of the most recognizable monuments in India. It is a 400+ year old mosque, though it is now mostly shut to the public (suicides?) and can only be seen from outside.
Biryani: Rice layered with meat (Lamb or Chicken) and herbs/spices. Places like Paradise, Hyderabad House and Bawarchi sell among the best, however, the most authentic is made in some of the Muslim families, which have been making it for centuries.
Pearls: Hyderabad is known for its pearls. One can get them in most jewelry shops, or head to Laad Bazaar at Charminar.
Irani Cafes: Pretty much any corner side chai shop. Highly iconic.
Haleem: During the fasting month of Ramdan, the famous Hyderabadi Haleem is made and available throughout the city. It is maily made of meat (Variants- Chicken, Mutton & Beef) & other delicacies. The Haleem that is available here is quite unique and a trademark of Hyderabad.
Other places of significant importance:
Tombs of Quli Qutub Shah, Golconda Fort (this is where the world famous Kohinoor Diamond was excavated from), Salar Jung Museum (the Nawab of Hyderabad was once the richest man in the world), Falaknuma Palace.
Opinion: The HiTech City in my opinion isn't so Hi Tech.