Chadwick Falls is Shimla's easiest nature break when you want forest rather than Mall Road. The waterfall drops through deodar and pine near Summer Hill, and it is at its best in and just after the monsoon, when there is enough water to justify the walk.
Go in daylight, wear shoes with grip, and do not expect a manicured urban park. The appeal is the short escape: damp stone, trees, birdsong, and a cooler pocket of hillside within reach of the city.