Here is one option
sun- drive LV to Zion. If you have time (depending on arrival time and car rental you could take the drive over to Valley of Fire just north of LV. It is a great little park and would be a nice intro. You really have more time for it at the end of the trip, but if you can do it first I think it would be better. (Plan 3-4 hrs for it). LV to Zion is 3 hrs, you will change timezone and lose an hour. Stay Springdale,,,lots of options, Desert Pearl is the top, Zion Lodge in the Park would also be great.
mon- zion Visit the Visitors Center if you are in Springdale. Drive through the park to the end. Stop first at the Court of the Patriarchs for the short climb above the trees to see them. Then go all the way to the end and take the Riverside Walk. On the way back stop at Big Bend, Weeping Rock, Emerald Pools lower. If you do it right it will be mid afternoon. Visit the lodge, see the history museum, if you started at the Zion lodge then come back to the visitor center.
tues - zion in morning, take another drive through the park. Maybe walk abit along the West Rim/Angels' Landing trail. Even a little ways up will give you some nice perspective. Then head east on hwy 9, just past the long tunnel take the Canyon overlook trail (no sign, just park as soon as you exit the tunnel, trailhead on the left). Enjoy the drive along the east side of the park. Stop at RedCanyon, then head to bryce afternoon. You should have time for a stop at the main amphitheater, each view point is great. Stay Ruby's Inn or Bryce Grand.
wed- bryce Take the scenic drive to the end, take the Bristlecone Pine trail, enjoy each viewpoint on the way back. See Bryce and Paria viewpoints. Come back and hike down Navajo trail. If you are up to it take the trail at the bottom over to Queen's Garden and hike up, walking along the rim trail to make it a loop. Stay Bryce
thur- Bryce to LV is unfortunately a 5 hr drive. But you can break it up by stopping at Valley of Fire if you didn't before. Or take the long way around by taking hwy 14 to Cedar and then west on 56 to Enterprise and then 18 south to Snow Canyon. It is a great little state park - still red and white rocks but with the volcanic black mingled in and unique in its own right.
I love the last minute trips, Have a good time and let us know how it turns out.