A bench, to sit on after a shower? You know, to make it easier to dry your toes, put on shoes and socks. A place to put your clothes, that you put on after your shower...when I suggested a bench to my dh, he was very enthusiastic. We've always had to sit on the (closed) toilet or use the bathroom counter for clothes - neither satisfactory.

Not a bad thought, but we actually have a chair in our bedroom for that exact purpose... Though 90% of the time it's covered in my clothes ;)

Upholster a piece of plywood and place it on top of the deck, and add some pillows that will hide the controls and spout. You've got an instant window seat, and yet it still preserves it's functionality as a tub for down the road. You may not want a tub, but many many people do want one in a master.

Yes, window seat with storage--it would be a great place to store towels or extra toiletries, etc.

Window bench, only if it has needed storage. Or a vanity with triple mirror and small bench - a place to sit and dry your hair or put on makeup.