I vacationed in Utah last week with my family - Zion and Bryce Canyon National Parks. Both were incredible. April is a great time to visit Zion: all services are open but it's not too crowded or hot yet (only 80 deg F). April is a little early for Bryce (at 8000 ft vs. 4000 for Zion) because some hiking trails were closed due to ice, but the low number of other tourists was quite nice.
Zion won in the things-to-do category. Next time we go, I want to rent a drysuit and hike up the Narrows, where the river is your trail between two steep canyon walls. We tried it with just shorts and sandals, but the 47 deg F water was too painful after just a fraction of a mile -- not a surprise. The most amazing sight was watching rock climbers who were just colorful specks on a vertical 2000-ft cliff face.
Bryce won in the photogenic category. You can easily see the highlights of Bryce in a day, sticking to the Amphitheater area. We spent a second day seeing the rest of the Bryce sights, but the kids were burned out on hiking so that got repetitive (in and out of the car at each scenic overlook).
We spent a little time in Salt Lake City, but not enough to really get a feel for it. We hiked up the east foothills by the Natural History Museum, which was quite fun (no other people at all)