Yes roads are good there , you can easily enjoy. And In Rishikesh, Nainital, Dehradun, Massoorie all places easily accessible with car. Like Laxman Jhula, Neel garh waterfall in Rishikesh, Nainital lake, Bhimtal are good to see .
Dear friend UK Is An Heavan Of The India and The Country Of Lord Shiva I Suggest U To Start From Haridwar Rishkesh Then Chopta kufri Nainintal There Are Lot To Go Just The Best Way To open The Google Map And Start Explore The Destination Which are Nearby U Or The Place Comes In The Route Best Of Luck For Us Journey!!!
6 days is less time to cover all gorgeous places in Uttarakhand and most of your time will be spent in only driving. It’s better to cover places nearby dehradun first or those in the same route like Rishikesh, haridwar in one direction and mussoorie , Uttarkashi , harsil and back in another.