I don't think you have to spend a whole day on Lantau Island. In the morning, you could take metro from Tsim Sha Tsui to Tung Chung station. Here, suggested you take Ngong Ping 360 to visit the Giant Buddha. Then, it should be around 12:00 when you return to Tung Chung station. You could have a dinner at CityGate just located here. In the afternoon, you could go to Disneyland directly by metro and stay there until to night after the fireworks.