Go to Old Fort on the top of a ruined structure there, you'll get beautiful views of the city along with Yamuna during cloudy and rainy weather. Otherwise you can also go to Rajpath.
Qutub Minar
Safdarjung's Tomb
India Gate.
Red Fort
Purana Qila
Humayun's Tomb
Isa Khan's Tomb
Lodhi colony
Well I believe that every monument and garden seems beautiful in delhi during rains. A drive in Lutyens delhi and cantonment area can also be beautiful.
You should definitely visit Deer Park and Lodhi Garden.
I'd advise you to go to The Tea Room in HKV. From its window, lush green deer park is visible which is the best thing anybody can imagine in monsoon.