Too much to cover in 5D and 6N.
Kullu is before Manali so plan Kullu first.Then travel to Manali.In Manali You can hire taxi from taxi station near the bus station and it will take you to Manikaran and Solang valley.The city of Manali can be covered by hiring an auto.December is very cold,days are short and there is always chance of rain/snow so many areas like Manikaran,Rohtang will be out of bound.