Bir Billing is the one which is packed with so many different adventures that you won't regret going there. Go to Bir by bus and then either trek to Billing or go by car, stay there for a night and enjoy the view with bonfire. Paraglide back to Billing the next morning. Hire mountains bikes the same morning and travel to the nearest monastery. Take a bus to Dharamshala the next morning, stay there for a day and then come back to Delhi. You can also directly go to Dharamshala, stay there for that day and then trek to Triund from McLeodganj. It's a very simple trek and you won't have problem reaching there. Stay there for a night and then travel back the next day. You'll be astonished to see the view up from Triund. The next one is Rishikesh. When I was there, I rented a bike the same day. Went for rafting the next morning. Took out the bike on the next and travelled to Mussoorie. You can travel back the same day or you can stay there for a night.