If you're looking for trees, probably the most heavily wooded section of the trail between St Charles & Hermann is just east of the
Weldon Spring trailhead. Nice views of the river too, and a nature walk trail at mile 53.5.
There are no scenic overlooks - where there are bluffs along the Katy Trail, the trail runs along the *bottom* of the bluff. But if you want a nice view from high up, ride up and across the
Page Avenue Bridge at mile 42.8 in St Charles. Nice bridge, great river view.
For historic sites, here's a general list of
"points of interest". Within this list you'll find historic sites and similar places.