Jim, just my opinion, but for a single day trip I'd do Zion. My reasoning is that you can see much of Zion from your truck and the pullouts, plus the shuttle into the rest of the park. We also loved Bryce but spent a lot more time there. We did several of the hikes down, and back up. See our website for info about that. You can see quite a bit from your truck but you'd need more time to really get to see that park. It would still be a good drive through, though. Again, just my opinion.
Jim, when going from Hurricane to Moab, you're not planning on towing through Zion, are you? You can do it but you have to stop at the tunnel and pay to have someone stop traffic and take you through the tunnel. And, of course, I think they do that like once per hour, so you might have to wait awhile.
If you go up around to Utah highway 12, then east, you could maybe drop the trailer somewhere near Bryce and do a quick drive through that park. UT 12 is a beautiful drive. We towed across there in the reverse direction. If you have time, going that route, we dropped the trailer at the CG parking lot at Capitol Reef NP and did a drive through (no trailers allowed beyond a certain part). And we towed through and stopped at the pullouts at Natural Bridges National Monument. Just some ideas for your "spare time!"
Or, if you're going south and around to Moab, you could easily drive through Monument Valley (fabulous) and, if you need a place to stay overnight for free, consider Goosenecks State Park, just west of Mexican Hat, UT. Fabulous place. Several MOC'ers have stayed there. Here's Robbie and Alice's rig at Goosenecks SP. It's all boondocking, though, but wow.
Enjoy. You two certainly are seeing some gorgeous country.
Tell Robbie and Alice 'hi' for us.