Last year I visited bir billing for paragliding by road. To explore Himachal roads are the best way to explore ,If you are living in north region of India otherwise take a flight near by airports. You can visit Himachal Pradesh any time in the year. Each destination has its own beauty which gives you memerising views and memories.
Himachal is a big state and weather is not the same everywhere. But to answer you question, if you don't mind crazy cold and snow then visit around Dec-Feb. March-May and then Sept-Nov its very pleasant(I prefer this). Rest June-August, it rains heavily(not recommended to travel as there are landslides in some areas).