The quickest and fastest route would be I-15 to hwy 20 east (just north of Parowan). THen to hwy 89 south to hwy 12.
Unfortunately this is also the best way to access the Kolob Fingers section of the park. You will pass it just south of Cedar City. If you have an extra hour I would highly recommend you see it at this time.
From Bryce to Zion your best bet would be hwy 12 to hwy 89 south to hwy 9. While the rest of the drive is not specifically scenic, this will get you into Zion via the east entrance. It is a beautiful section of the park and not to be missed.
If you want instead to go back to see the Kolob Fingers section then take hwy 143 out of Panguitch to 148 (through Cedar Breaks as long as it is open) then hwy 14 west to Cedar City and then south to Kolob, then to Zion via the south entrance. If you do this be sure to take the drive out all the way to the east entrance while you are visiting.
If Cedar Breaks is not open (depends on the snowfall, there could be the possibility it is open in early May this year). Then just take hwy 89 to hwy 14 to I-15.
If you plan on visiting Kolob Canyons, you really need to visit en route from Las Vegas to Bryce, or from Bryce back to Las Vegas. It doesn't make any logistical sense to make a trip there and back from either Bryce or Zion. The visit to Kolob can take less than an hour if all you do is drive the scenic route, and take minimal stops. Parks passes bought at Kolob are good for seven days, and will get you into the main Zion portion of the park, or vice-versa. As Kbec mentioned, driving the Mt Carmel hwy/SR 9 is an essential and recommended part of any Zion visit.
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