What are some ways to "hide" a window behind a bed?

Ive done this. Pop in some drywall, you can get it thin or standard, and paint or wallpaper it to your liking. It will look like a headboard. Feel free to tuck and pin fabric instead and the window frame will turn into a headboard board frame.

How about putting in some wall to wall curtains behind the bed and headboard?

If possible, turn your bed around. Put a dresser or chest of drawers in front of the window if its existence bothers you. Try to look at your room at a different angle. You may like a change. Best wishes.

In my previous home. I put small boards across the top and bottom of the wall that had the window. Then I gathered yards and yards of inexpensive fabric and stapled it at the top and bottom keeping the fabric straight and taunt. I stapled it at top and bottom and then added crown moulding at the top and base moulding at the bottom. I found a bolt of fabric in a color I wanted to use for the draped wall and then took it to the paint store and had the color matched with paint to paint the other three walls. I also stapled heavy duty insulation drapery backing around the window so that the sun didn't shine thru. With the board at the top and the bottom, the fabric was out enough from the wall that you could not see the framing of the window. Good luck.

I just used my hand held vacuum to clean it when needed.

we put foam board inside window frame to help block light and insulate, then hung a curtain from very close to the ceilng to down behind the bed.