If you have 3 days of time I would suggest Tirthan Valley is a must.
And for other weekend trips u can go to Parashar Lake, Lansdowne, Kasauli, Rishikesh, Chail, Mussoorie, Pauri, Nainital
Almost all of Uttrakhand is unexplored, small towns like Almora, Dhanaulti, etc. are paradise for nature lovers and you can stay there for amazing cultural experience too