Dehradun as I would call it is one of the best weekend connection from Delhi. Connected with Delhi by road and rail, one can easily reach Dehradun by their own car (248 km drive approx) or take buses (online booking of Volvo available on redBus, Uttrakhand tourism website, UP Tourism website) or by train (Shatabdi Express, Jan Shatabadi Express, Mmussorie Express, etc).
I have always preferred to self drive through NH 58. The road is in much better condition now and makes the drive comfortable. Passing through Khatauli, Roorkie, one has the option to reach Dehradun from Haridwar as well (which is 40 km drive in addition to 248 km) or straightway take left from Roorkie towards Dehradun using the bypass.
The drive from Roorkie to Dehradun passes through the hills and forest and is awesome drive if the weather is good. However it is not advisable to drive during night because animals like Deer, Elephant, Fox come out on the road so better be careful.
If on self drive do not miss pakoras at Moolchand, milk shake at Roorkie Chowk. If you are on sightseeing trip, you may visit Shakumbhari Devi Temple, Daat Devi Temple on the way. You may also route your trip from Haridwar and add Mansa Devi Temple, Har Ki Pauri, Chandi Devi Temple, Rishikesh, Neelkanth Temple to your sightseeing trip before reaching Dehradun.