see + do
If you can arrange it, try to stay overnight at Aguas Calientes so that you can catch the first bus up in the morning. Sunrise over the Andes is stunning, especially when you have Machu Picchu to yourself (you and the llamas mostly). Adventurous? Try the hike down to the Door of the Moon, into the rain forest, under rocks and with no one else on the trail. The sunrise visit and the Door of the Moon hike should take you until about lunchtime--plenty of time to ride back down and catch the afternoon train to Cusco.