In Zion the 2 mile round trip Riverside Walk is my favorite. It's wheelchair accessible! My second favorite is the Lower Emerald Pool trail. If you can, I'd suggest at least going to the Middle Pools but the Upper pools are strenuous! Weeping rock is short but has some slope.
In Bryce I like walking along the rim! The Navajo - Queen's Garden loop is about 3 miles but I don't know if that would be considered a moderate slope or not. Go down the Navajo loop...you can see the switchbacks from the overlook to see if it's going to be to much. Rangers said to come up the Queen's Garden because the slope was less.