Har ki dun trek is a good option.
you can also plan for chopta trek in Uttarakhand and nag tibba trek best way to check for upcoming treks and other knowledge on trekking visit INDIAHIKES website in which you can get great help. other treks to be considered 1: Phulara Ridge trek (easy level)
5-6 days 2: Dayara Bugyal trek (easy -moderate)
5-6 days 3: Chadar trek - frozen river zanskar trek