Hey tanzil
Actually it depends on your interest, solo trip can be anywhere where you feel good it doesn't need to be just mountains or just beaches.
If you are starting with karnataka then try some places less crowded i would suggest Hampi but now it's a UNESCO World Heritage site so it's getting crowded day by day.
If you like mountains try Kodaikanal...chikamanglur
if beaches then go to some silent beach where you can enjoy sunset like Mangalore
Gokarna.
An amazing place to chill and hangout with different kind of people.
Delicious South indian karnataki food.
Beaches and outstanding views.
Beach trek.
A short road trip to vibhooti falls and Yana caves.
Spiritual visit to Murudeshwar temple.
And last but not the least the Trippy local bhang called a Magic lassi at Sea rock cafe on Om Beach
For your first solo trip i suggest you to visit himachal as it is cheap 1 week you can spend 7 k not more than than and can cover many places nearby . as it is famous for beginners trek
I would suggest you to Start from Gokarna,they Marudeshwar to Mangalore Bus trip(must be memorable one,take stop at Thrasi,Udupi,Surthkal for beach view)
crystal iceland in Mangalore then move to Mysore then Bangalore and Hampi.