If there's one thing I miss about living with my mom it's the bay window she has in the family room. Lots of butt room and potted plants and sunshine. Of course, being self-banished to the basement whenever anyone else was home, Holland would only use it when we had the house to ourselves...
I have always had a kitty window perch in atleast one window, it's a kitty requirement! Maia has a carpeted window perch with a carpeted ramp up to, and a second 16 inch higher shelf, this is her window!
Our house doesn't have window stills. We have the catnapper in the bedroom for them and we have the bay window in the living room. When there's something outside they really want to see, they manage to fit in the window when the windows are up and the screen is there. I've often said to Rick that I would love to have sills wide enough for kitty butts in all of our windows.
How about a bookcase under the window and level with the sill? Jamie has window perches in the living room and kitchen, and spends most of his time there in the winter. Our sills are about 5 inches deep, and the perches are attached underneath them with a screw, but there are ones with supports that rest against the wall.
One of the things we looked for in our current house was wide, low sills. The cats love them. Of course, we have to leave the blinds up a foot or so, or they'd wreck them trying to see out.