I think for the large wall I would look for a pretty wallpaper. That's a lot of space to fill and wallpaper could do it for little money.

For the other room can you move the bed to go between the two windows?

Sometimes two twin beds in a guest room can be nice, especially if its a second guest room. Two twin beds are separated so they can actually take up a large wall. I'm not sure if this fits your exact arrangement, but is just one suggestion. Another suggestion for if the room is big enough to accomodate it is built in shelving.