Tungnath trek (chopta) nearest
Har ki doon Chadar trek But Don't go in these days. It is raining and snowing heavily there.. You can go to Pune , goa or Munnar for treks and beautiful sunsets/sunrise.
Nag Tibba is the nearest trek from Dehradun which can be easily done in 2-3 days. The trek is of easy and you can return in 2 days. To know more about this trek you can read this blog - https://www.bizarexpedition.com/blog-nag-tibba-trek