It should be Manali>>Kasol>> Tosh>>Kheerganaga trek Why the above route plan is the best because in December the temperature will be the lowest and the whole Manali town will be covered in snow.
Kasol is the ultimate place for bachelors (if you are), the riverside cottage houses are the ultimate place to find peace. If you smoke weed then this is the place for you. The place is serene and crowded with hipsters and solo travelers, the young soul rejuvenates there. Tosh - I am not sure if you will be able to trek to Tosh in December. I have not been there at this time of the year. But still, even you can’t trek to tosh the sightseeing will be treated to the eyes. You have to keep a couple of days for Kheerganga, taking into account the exhaustion from the trek uphill and downhill. You would need to either camp up in Barshaini or Kalga\Pulga to start your trek. Barshaini is around 18kms from Kasol. Kalga is 4 km uphill from Barshaini. My suggestion is to stay at kalga and get acclimatized and start your trek from there.
You can take either of the following route:
Delhi - Manali - Kasol (with Malana & Tosh) or in riverse
Due to snow, Kheerganga Trek will not be possible, to visit Kheerganga, you can go in Summers or in December, instead of Kheerganga, you may add Manikaran sahib, hope this helps...