Stop at a clean highway cafe or pack breakfast from Andheri (recommended to pack idli/dosa or parathas). Saves time and money compared with hotel breakfasts.
Check in to a hill hotel/guesthouse (2 rooms assumed). Expect higher prices in December; prebook. Comfortable, centrally-located hotel makes Mall Road access easy.
Popular waterfall ~15–20 km from Mussoorie; best visited mid-morning to avoid crowds. Typical visiting hours ~9:00am–5:00pm (confirm locally in December).
Start the return with a long driving leg; fuel up, and plan driver rotations. If you're aiming to reach Mumbai by night, be prepared for long hours or split the return with an overnight stop.
You may reach a city such as Delhi outskirts or a central rest point for the night — adjust booking as needed. If you must be home that night, plan for 14+ hour driving or drivers/naps.