Lived near shimla. Frequently visited there during my stay in Himachal. My highlights were:

• Walking on mall road. We would walk 2-2.30 hours continuously with friends. Had a term for it "Gedi Marna".
• Momos at Krishna Bakery, mall road
• Chadwick Fall, you should visit it. A small water fall after a 15 mins trek. Generally, no one is there. If it's not cold and not much water, you can have a natural shower.
• Heritage Toy train: Some breath taking view during it's 4 hour Shimla to Kalka journey.
• Went to St. Bede's college for a cultural program. Hands most beautiful place in Shimla (-:
• Don't visit Kufri. Too much hype and no fun.

There are amazing places to trek and camp around Shimla. Spend a day in Shimla and hike some beautiful trails.
*flies away*