Hi Manish, some of the popular hill stations near Delhi are Bir-Billing, Kasol, Dharamkot and Khajjiar. If you want to go to slightly off-the-beaten-path hill stations then the options are Kausani, Naukuchiatal, Gushaini and Parwanoo. You can also refer to the attached article for more hill station options near Delhi.
My personal favorite is Bir Biling. We went there for 3N4D trip and we indulged in a lot of activities some of which are Para gliding, trekking, camping, biking etc. It was one of the best experiences I have had in hill stations. We went there in June and it was freezing. The views were breathtaking and chilly winds were cherry on the top :D P.S. Do let me know your experience wherever you plan on going..