From Shanghai to Huangshan, it will be best to take the 5-6 hour bus ride. Trains going to and coming from Huangshan take longer.

A weekend trip is enough, but it will still depend on what one wants to see. The International Youth Hostel in Beihai Road in Tunxi is highly recommended. They can also take care of the bus from Huangshan city to Huangshan gate (1.5hours) and the bus from Huangshan back to major cities (Beijing, Hangzhou, Ningbo, Shanghai, etc.)

If you decide to spend the night in one of the hotels in the peak, you will need to book a room in advance. One advantage of staying overnight in the mountain is viewing the sunrise the following morning. Do not miss Tiandufeng.