I think some bright dishtowels and curtains would really brighten the area, and it looks like you could put a small picture next the window. I think your kitchen looks cute the way it is in the picture though.
If the kid is young you could make a felt board with his favorite characters on it (I made a Spongebob one for my nephew). Blankets and pillows are a good idea for guys of any age, you could even base designs off of their favorite games.
You can use old grocery bags to pad the xbox, it's cheap and a good way to make sure things don't break. I'm not sure what you mean by securing the box to a footlocker, but I am fairly sure industrial velcro exists.