If you decide to go to the Bryce area, and the weather is not too hot, don't miss Kodachrome Basin State Park -- great hiking, great slot canyons, very nice campground (with full hookups, amazingly). This is an overview of part of the park:
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Here is a blog link, if you'd like more info -- this post contains some detailed trail descriptions:
Bryce, Zion, and KodachromeTwo more points about Zion: if you are a bicyclist, the canyon road is terrific -- long, gently uphill, long glide back down. For much of the year, cars are not allowed in the canyon -- I am not sure about September -- but when cars are banned, the shuttle buses are very courteous to bikes.
Second, we have never done the Narrows hike, but we saw quite a few folks who had obtained equipment for that hike from outfitters in Springdale -- wetsuits, special shoes, etc. They all said that it was worth renting the equipment. I am not sure whether you would need all of that stuff in September, but it was necessary in November -- quite chilly at times!