This year March is the best month for Manali as ATAL TUNNEL is open and you can go till sissu. Stay in Manali (I will recommend Acharya cottage. Great host, great food and budget stay).
Go to Sissu. If you can go beyond, visit jispa, triloknath. It will take 2 days to cover sissu, triloknath and jispa. If you want to go till issu only book a stay there and enjoy the landscape.
Try humpta pass (1 day)
Roam in old Manali in evening.
Visit Bijli Mahadev (1day)
Anjani Mahadev is also an option ( covered in way to sissu).
Malana Kasol (2 days). You can have a simple cottage with kitchen for around 12000 if you are a long term stayer or a month stayer, work from home person.
Just Go on Goibibo site/app and book your tickets to delhi from your destination and then book a complete package for manali / Book a volvo from delhi to manali. Goibibo give you best hotels in manali. Street food is very delicious in manali market.
About 3-4 days should be more than enough. You can resrve an entire day to visit the Rohtang pass. Another day can be reserved for visiting the Hidimba Temple and the Mall Road. The other two days can be kept for leisure.