Went to all of the parks when I was young. The rest of my family preferred Bryce, for the pretty scenery, but I preferred Zion, since you were in the midst of the canyon's attractons vs. looking down on them. I see you went to the Grand Canyon, but if it was the South Rim, you might consider the North Rim. A very different experience: 1,000' higher, cooler, greener, much more forested nearby, and far less crowded. However, far fewer options for accommodations: the lodge on the rim, which sells out way in advance; the rustic Kaibab Lodge just outside the park, the the Jacob Lake Inn-- essentially a big motel-- 40 miles from the rim. If you've been to the North Rim, and don't have time to hit the South Rim this time, maybe hit Capitol Reef National Park
http://www.nps.gov/care/index.htm
The last time we were at Zion we stayed at the lodge inside the park, since even hotels in nearby Springdale were largely sold out. Glad we did, since we had the area to ourselves after the day's last shuttle bus to Springdale left and before the first one of the day arrived.
Are you going in August again? Obviously warmer (especially Zion) and more crowded than, say, after Labor Day. Facilities at the North Rim of the GC close in mid to late October. I think similar dates for Bryce, even if the road is still open.
As noted elsewhere, there is no Moab National Park. Maybe you mean Arches National Park. Very scenic.