Bikaner if you are a desert person
Sikkim if you are a quiet river person
Bhimtal if you are a mountain and adventurous person
Gokarna if you are a beach person
It depends on 1. place where you live
2. budget
3. interests My favorites are 1. Kasol
2. Old Manali
3. Rajasthan (any place in the state)
4. Diu
5. Pondicherry and so on.
I have been to a lot of solo trips. Personally I suggest you to choose a place which is like 200-300Kms away. I started like this and suggest this to everyone, once you get use to travelling alone, take long trips.
Depend on you interest. You can explores beaches of Goa and Konkan,. or go to solo trip to Rishikesh and above areas. I went both kind of places and it's awesome and safe. Check my beach vlog on YouTube.
https://youtu.be/_BbE3UJZXlY