My family and I just returned from a 12 day trip to Peru.
One day at Machu Picchu just will not do it in my opinion. The reason is that you will arrive at Machu Picchu at about 11am (train takes 3 1/2 hrs from Cuzco and bus another 30 min between Aguas Calientes (where the train stops) and Macchu Picchu itself. This means you, and every other tourist, arrive at the same time. It is a zoo.
If you return to Cuzco the same day, you will have to leave Machu Picchu at no later than 2:30 pm to catch the train back. So you will have at most 3 1/2 hrs at the site.
To really enjoy it, you need to plan on spending the night at Aguas Calientes. That way you have an almost deserted site to explore from 2:30 till 5:30--it is wonderful at this time.
There are also many other sites which are close to Cuzco which are worth a visit.
Another consideration is that if you just go directly to Machu Picchu, you will not have allowed any time to acclimate to the altitude. I saw many people who were having difficulties because of this.