There are a million and one things you could check out but having the time to do them is something that most of us struggle with. Sometimes less is more and just enjoying the places you are visiting rather than rushing to the next (very tempting I know !!) is often more rewarding. You also have to remember travel times when planning a day here and a day there. For example, by the time you get to Bryce from SLC thats going to be the best part of half your day gone. Bryce is quite an easy park to see the best parts as all the major viewpoints are a short way from the roadside. If you travel to the far end of the canyon and sightsee on the way back all the viewpoints will be on your side of the road and you won't have to constantly cross traffic. Zion is different, there are loads of easy hiking trails and it takes longer to get around, thats why I suggested an extra night there as 2 nights means you get a full day to explore the park but also allows you some free time to travel in and out of the park to and from your other destinations. The simple journey from Bryce to Zion will have you wanting to stop at places like Red canyon, Checkaboard mesa and maybe even the Canyonoverlook trail.