Great thanks for the comments. I'll keep the crowds and the transportation restrictions with the shuttle in mind.
Hi and welcome to Zion NP! I am co-owner of a photo gallery in Springdale, a mile from the main gate to the park and hhave spent a lot of time there. I agree with those who say you won't have time for any serious hiking on a one-day trip.
When you arrive from Las Vegas, park your car in Springdale. Do not try to park in Zion. There are plenty of places all over town. Walk out to the main street, look for a shutle stop (well marked) and take the (free) shuttle into the park ($25?) where you'll switch to another (free) shuttle that runs only within the park. This shuttle will take you along the Virgin River, up the deep valley floor to an area where you can walk a mile to the beginning of the Narrows hike. There are many stops along the way where you can get off the shuttle to look around and take pictures. Another shuttle will come along in less than five minutes to take you onward/
I would sugegst that you then return to your car in Springdale with more stops as you wish. Now you want to drive your car back into the park to the bridge over the Virgin River where you'll turn on Highway 9, to the east. This willl take you up to a 1.1 mile long tunnel built inside a cliff face, and on into the rugged upper section of the park (it doesn't have a name), which many think is as spectacular as the lower canyon of the Virgin that you will have seen earlier from the shuttle. On this road there are many places where you can stop for a picture or a walk.
The sun is setting here now at around 9 pm. In September you'll still have a long day so you can judge how much time you have before darkness. If the moon is full, enjoy the park in the moonlight. It is an awesome sight!
For a late dinner, there are several nice restaurants in Springdale.
I recommend that you go to a map store in your neighborhood and get enough maps of Zion to familiarize yourself with the layout before you arrive. After you arrive, there are excellent resources at the park headquarters. You'll go by there when you make the shuttle transfers.
Come back in November when the leaves turn and you can drive your car up the valley road!
If you'd like more details, email me. Have fun!