Himachal Pradesh is not one destination. Shimla and Manali are easy family hill breaks, Dharamshala and Palampur are quieter and greener, Spiti is a high-altitude cold desert that needs a proper acclimatisation plan, and Kinnaur sits between the two. The route you pick matters more than the number of days.
Chandigarh is the main gateway by air and rail, with a four to five hour drive to Shimla and eight to nine to Manali. Kalka connects by the narrow-gauge toy train to Shimla. Overnight Volvo buses run from Delhi. For a family or group, a private vehicle from Chandigarh is usually more comfortable than changing buses, and it lets you stop at the places worth stopping at.
Mountain driving is slow. A 200-kilometre day in Himachal can take seven hours, so an itinerary that looks reasonable on a map often is not. Carry cash for smaller towns, keep a day of buffer in the plan for weather, and if you are going above 3,000 metres, build in acclimatisation rather than driving straight up.
See the Manali tour package, or SUV bookings if you are driving the circuit yourself with a hired vehicle.