Get a National Park Pass Just do it
I agree dont miss Zion but unlike Paul if you have only one night you will really benefit from staying at ZIon Best Western was a fair rate for what we got, killer view and being right there
The free bus to the park is right about 100 steps outside the door and lots of good dining,
We made reservation well in advance rode up in the rental car arriving by 10 or so, room was ready and we dumped the overnite bag and took the bus into the scenic area then hiked till dinnertime Next day we rode 9 both ways then returned to Vegas

You cant drive the scenic trail during peak season but it was open in March when I was there on the Valk the free shuttle from the entrance will take you everywhere and you can hike to your heart's content You can however drive rt 9 thru the park and then swing up to Bryce or wherever