According to your budget as you tell me about, you have to go Nainital.
A city with Amazing natural beauty and low expensive Environment where you can enjoy with your parents and give them a beautiful Surprise also with minimum budget. here many of affordable packages for nainital for you is http://packagestour.com/nainital-tour-packages/
You can visit Dharamshala, Gujrat,Udaipur,etc. Avoid travel in cities like Mumbai, Dehradun, Nainital, South India etc. as monsoon will not let you enjoy your trip.