Hill Stations of Himachal Pradesh are best to visit during May. You can visit offbeat places of Himachal Pradesh like
Jibhi
Tirthan valley
Barot Valley
Janjheli
Shikari Devi Khajjiar Dalhousie
I would like to suggest Rishkesh for pritine and fresh waters coming from snow melt. because after the first rains in June, the water is filled with mid that comes from mountains. Even though you might find it a little hot in weather because its not a hill station but its kind of off season to try certain experiences and do a budget trip.
May in India can be a bit tricky weather-wise,these are my top picks-
Himachal Pradesh: Shimla, Dalhousie, Manali
Kerala: Munnar, Alappuzha, Coorg
Goa: Palolem Beach, Calangute Beach
Ladakh
Meghalaya For more in-depth recommendations, check out this travel blog - https://tibro.in/blog/30-best-places-to-visit-in-summer-in-south-india/