At dhanaulty camp stay is a must. from where you can see different shades of Himalayas at different time intervals very clearly. I can recommend you some camps there. you can see my photo blog for details. rest if u need I can share you the connects there.
Best way to this trip is Reach Dehradun and take a bike or scooter on rent which is easily available there. Reach Mussoorie First day and leave for Dhanaulti around 4 PM so you can reach Dhanaulti by 5:30-6 PM in way you can see beautiful sunset from some points if you are lucky with time and location.
Stay night at Dhanaulti and next day enjoy morning there. now if you want to visit tehri and return back same day then leave around 10 AM and you will be at Tehri by 3 hours if you enjoy the way otherwise you can cover distance in 2 hours also. Now spend time there and leave before 4 if you want to get back at Dhanaulti and can leave at 5 if want to stay at Chamba. You can do same trip with public transportation which is available there but frequency after Mussoorie to Tehri is very low by Buses.