should set a bit before midnight when you get there, so be sure to stay up late for the best star gazing. If you are near Cedar City on Monday there is free weekly public star gazing at the Southern Utah University observatory.
Fall colors are looking nice now. We drove near Bryce for a cross country meet today and it's beautiful in those valleys and just getting better. Fall colors are different from East coast given the desert climate, but still an interesting contrast against the cliffs and rugged mountains. Zion will also be nice, and consider Cedar Breaks for a relatively unknown national monument as you travel through.
What activities are you planning? There are bike rentals just outside Bryce and more southern Utah hiking trails then you could manage even with years to explore. Bryce will have cool to cold fall weather and St George should have perfect weather. If golfing, plan on St George.