Tapovan is another temple that you can head to. Mythologically, this temple is known for being the place of penance for Guru Dronacharya. And here's some good news for the adventure junkies out there, you can trek through the trail located above the Gangotri river to this temple. Tapovan is 5 kms away from Dehradun and there are plenty of cabs, or buses that go to and fro from these points.
You can also head to Tapovan ashram, where you can be a part of spiritual activities including yoga and meditation classes. This is basically a place for people who want to get away from the city's hustle-bustle, and there's a lot of greenery and calm surrounding the area.