I didn't like Dehra Dun. Don't go to Dehra Dun if you don't have anything in particular to do there, because it's no good place. The streets are dirty, rickshaw drivers try to fool you, hotels are more expensive than usual, the city has a bad smell in general and there are lots of beggars. Especially children.
I went there because of the Vipassana camp and I left real quick to a beautiful place 15 kms from there, which I told you about in my Vipassana article. (only available in Hungarian, still, sorry).