Hi, December will be freezing in Manali. The best treks would be Beas Kund, Hampta Pass or Chandrataal Lake, but you need to check if the trails are open and safe, according to the weather conditions :)
You can try out certain short treks around Manali during winter time apart from what has already been mentioned below by fellow traveller @Riyanka. Such examples are Prashar lake trek, Bhrigu lake trek, Kareri lake trek. These trials should remain open (however all depends on the weather God).