We also made a trip from Vegas up to Zion and it was definitely worth the drive (about 2.5 hours each way). We went on a weekday in Early December so there we basically no crowds, which is pretty unusual for a park as popular as this one. The weeks before (around Thanksgiving) were so crowded that the road had reached capacity and they stopped letting people in. We only had a few hours and I'd love to go back for a longer stay but we were satisfied with what we were able to see.
We started by driving from the visitor's center to the Riverside trail. This is a pretty easy walk, we actually brought sandwiches from the store near the visitor's center and ate them on the walk. I think they have more options during peak times, but since it was such an empty day the only choices were premade breakfast sandwiches leftover from the morning, but they were discounted to $1 and were delicious! There is also a Zion brewery I'd love to check out sometime.
We were hoping to see more wildlife on the walk but we did see a deer. It was probably the wrong season, because other friends I've talked to have seen a lot of wildlife including bighorn rams and condors.
From there we realized we were running out of time and went right onto the Angels Landing trail. It takes a few hours just to get to the Angels Landing trail, and it is almost entirely uphill but the scenery is incredible. The pictures don't do it justice. I wouldn't recommend it unless you are in fairly decent shape. We thought the walk down would be easier but on tired legs walking several miles straight downhill is not easy!
Here you can see part of the trail leading up to Angels Landing (this is just the beginning part)
We didn't end up hiking all the way to the top because we were running out of daylight, and were pretty tired from the hike up already. The trail continues up the narrow section seen in the photos above, with just some chains to hold onto.
Even without completing the last part of the trail we were able to see some spectacular views.
We also wanted to hike the Emerald Pools trail but it was getting late and we had already hiked pretty far so we decided to save it for next time and headed back to Vegas. We made it back in time for dinner at Canonita in the Venitan. Probably the best tacos and quesdillas I've ever had and they had $5 (good!) margaritas!