Zion:
Canyon Overlook Trail: Parking is very limited but the view is astonishing and relatively easy trail. This is a great alternative if you can't make the much harder climb to Angels Landing.
Emerald Pool: Great if there when it's hot so you can cool off in the water though prettier in the spring with more water. If you are in good shape, you can do the trail in about 15 minutes each way by running though recommend a couple of hours to really enjoy it.
Bike It: If you have bicycles, catch the shuttle to the Narrows and then coast most of the way back to the campground. The incline is gentle so don't have to worry about your brakes and gives the opportunity to stop at all the pull overs the shuttles don't stop at. The shuttle drivers watch out very well for hikers and bikers so no concerns there. Only one short steep up hill coming back down which we walked our bikes back up and then after you cross the main highway the path along the water is flat. Did this with my two kids in 2005. I sent my 14 and 12 year old daughter on the first shuttle and then I went on the next shuttle. Driver was very helpful loading/unloading their bikes and even went a bit slower than normal to reduce the time between his departure and my arrival.
Bryce
Horses: Only thing I can suggest is take a horseback ride down into the canyon. Take the longer 3 hour ride otherwise you don't get much time down in the canyon. You will probably see a lot more than if you walk down.
Cave: Drive around to the north side of Bryce and take the leisure stroll up to Mossy Cave.
2015 Ram 3500 4x4 Crew Cab SRW 6.4 Hemi LB 3.73 (12.4 hand calc avg mpg after 92,000 miles with camper)
2004 Lance 815 (prev: 2004 FW 35'; 1994 TT 30'; Tents)