Get down at Himachal Pradesh station, take a bus to Kasol, explore the valley, Trek to chalal, join a camp there, live a night, return back to Kasol, take a state transport back to Himachal station.
and yes, don't miss the stuff ☘️
Use Himachal Pradesh (HRTC) buses normal or semi deluxe buses ,buses going to Manali are also an option ,get down at bhuntar and from there take a local bus to reach there at kasol ,try staying at hostels or book your hotel atleast 10-15 days back ,eat at local dhabas ,they are also very good ,and use local buses or shared cabs/taxis to travel
You can trek to Chalaal village, kheerganga (spend a night there in camp). explore grahan village in old kasol. Explore malana. eat amazing food at cafes or echeck out the amazing hill top view from jim morisson cafe. do not forget to try out amazing pie and cakes at german bakery.