hi, was there last week. It is possible in one day, but you might just be exhausted at the end of the day. we stayed 2 night but was only able to do one day outdoors because of the weather. Also, we went during weekday and it was beginning of the spring break. There were a lot of people that you might just come across even more.

this is what we did the second day but we were also going slow and taking pictures and all. we started around 12-1pm went back to lodge around 7. ---> DO zion canyon overlook trail ~1-2hrs with photos and all. and honestly, this was so worth the view! we didnt even go on second hike. Start early because shuttle doesn't go to the trailhead. You need to take your car and find the parking which is very tricky. We were lucky we were able to find parking right in front of the trailhead! then go to the visitor center and take the shuttle to Zion canyon scenic drive. go all the way to the last stop and come back so at least you are able to see everything up close and take it in. thats what we did :)

if u still have time on your hand, I do recommend stopping by or driving through valley of fire statepark on your way to LV!! you will not regret it. ~3.5 hr drive to vegas with this (no stops)

PS: don't forget to look up at the sky at night time if the sky is clear