The challenge is simple: design an autonomous or remote controlled window cleaner.
It might climb a wall and then scoosh high pressure water at the window. It might climb the window itself in the case of an all-glass building. It might be something you stick on having opened the window. It might use a squeegee or something else...
Umbilicals are acceptable for power, water, control, etc. Of course there is kudos for a self-contained device.
Naturally, there is uber-kudos for autonomy.
Some inspiration can be drawn from: