Tokyo to Kyoto daytrip

Sorry for all the questions here but:
Would it be plausible to spend a day in Kyoto and Nara if traveling from Tokyo?
According to this: http://www.hyperdia.com/cgi-english/hyperWeb.cgi site, I could grab the Shinkansen at 6:07am getting me to Kyoto for approx 8:10am, and then back again quite late into the evening.
If I were to book a return ticket, do I have to specify a return time or do I have the flexibility to return at whatever time I want?
If I could get to Kyoto for 8am I was thinking about seeing a few easily accessible sites and then if time permits it moving onto Nara and seeing Nara Park and maybe a few other attractions.
After doing some research it SEEMS to be possible, but am I being unrealistic?

It depends on your travel style! Some people do it. It basically means paying 30,000 yen and spending six hours on board of trains for a little bit of time in Kyoto. Some people find it perfectly okay, for others it is a crazy idea. It is up to your personal travel style.

However, if you decide to make the side trip, I strongly recommend to forget about Nara and only visit Kyoto. Also packing in Nara would add an additional two hours to your time spent on trains.

Thanks for the reply Uji, i've decided to spend the night in Kyoto and see Nara the following day.
I'll be able to see more of Kyoto this way and get to see Nara without rushing around too.