You can go to Himachal for Solo travel.
Volvo are easily available from Delhi and leave in the evening to Himachal.
You can do easy treks like Kheerganga, serolsar lake or Triund.
You can chill in kasol or Manali
Tamilnadu...and in tamilnadu if u have seen Chennai means then turn ur ride to South side like Madurai,tuticorin,kanyakumari and lot of cities are there for u this is first time to tamilnadu means choose some ancient temples occupied cities and sea shore cities so u may not get worried by ur trip.Near by tuticorin there is seashore place it is a best best best vocation place to enjoy..
South India is a good option as it is much safer for girls. Popular tourist attractions like himachal in north india attract people indulging in recreational drugs so not a safe option for a girl on a solo trip.
In north India, If you want to just chill & love hills
Manali, Shimla-Chail in Himachal
nainital, Mussorie in Uttarakhand if you want to see historical places & palaces
Jaipur, Udaipur in Rajasthan If you want to explore less crowdy places Sikkim, arunachal Pradesh would be good If you're into beaches a lot then
goa, daman & Diu
First of all, no trip is for girls. If the trip is fun, its ewallu for all. If you looking for safety options, travel agents and team like zostel has already sorted things for you. now to start with, You can plan for Thailand in less than 25K assuming you book the flight tickets on time. Bali, goa, rishikesh, kasol, nainital, are some of the places worth considering