Himalayan National Park near Kullu, Stay there and spend day 3 trekking in the park. Move to Manali for an overnight stay.
Another thing is you can go for rafting (which is one of the best I have done so far). Good Luck!!
Keep your phones fully charged when you leave from the hotel to explore the streets of Manali. Believe me: you do not want to miss out the opportunity to capture a wonderful moment on your camera. Check: https://www.tourpacx.com/best-things-to-do-in-manali.php
Good luck for your trip and do share your experience with us! Be safe and be happy!
Hey Abhishek, Offbeat things to do in manali are : Go Paragliding in solang valley Visit Nagger castle ... Trek to nyigmapa monastery ... Go to ROHTANG pass .... there is a river side cafe also to enjoy hot beverages ... River rafting in beas... visit hidimba temple and also walk amidst cedar and pine trees .... Roam around old manali ... You can do all of these by hiring a motorcycle in Manali ... Happy Travelling Bro!!!
Hey Abhishek, You can trek to Malana village of Parvati valley directly from Manali. First take a bus or cab to Naggar then from there it takes 2 days to reach Malana via Chandrakhani pass. If you have done trekking before then this is the best option I would recommend you. Happy travelling!!!!!!!!