Definitely all the known places - India Gate, Rashtrapati bhavan, Lutyens Delhi, Red fort, Humayun's tomb, Lotus Temple, 11 Statue, Jantar Mantar, Qutub Minar, Safdurjung tomb, all well Known markets, old Delhi, Gates of delhi, old fort, lodhi garden, India Habitat centre, Mall of India, Dlf place Saket, Select City walk, GIP mall, Cyberhub Gurgaon
Delhi has a lot of unexplored places as well. Like many monuments and forts and believe me they are really beautiful. You should visit that as well. You can google it. Rest you can visit the streets of Chandni chowk, take a morning stroll at India Gate, Eat from the streets of South Delhi. And what not. The list is endless!