I would suggest Dharamshala. Not only it has a trek in terms of Triund, you have beautiful caves, lakes and waterfall there. Also, Dalhousie is very near so you can visit it and explore Khajjar.
Hi, Ayan!
Places like Spiti Valley, Tirthan Valley, Gulmarg, Mcleod Ganj, Nainital, Mussoorie, Gangtok, Tawang, Shillong, Cherrapunji, Mawlynong, Darjeeling, Coorg, Kodaikanal, Munnar, Matheran, and Lonavala are some really great places to enjoy a holiday with friends in the month of May. These place offer amazing weather conditions and spectacular views of natural beauty that our country is home to.
You can also check out following Tripoto links to help you decide a destination to travel to with friends in May:
https://www.tripoto.com/trip/best-places-to-visit-in-may-in-indiahttps://www.tripoto.com/trip/best-places-to-visit-in-summer-in-india-56cc844facd72
Hope that helps!
Sikkim as sikkim have many off beat as well as popular places to visit during summer time. You may visit Manipur or Nagaland. Or else South India like Munnar or Alleppey. Or Uttarakhand like Nainital-Ranikhet- Almora-Kausani-Pithogarh etc. you'll get the best view seriously!