The city in which I live is clogged with litter. As far as I can tell, some moron gets drunk and decides that throwing crap out his car window is disposing it. Also, I occasionally find tires where somebody has decided that since disposal costs are so high, that the best thing to do with their worn out tires is to toss them where no one will notice. Someone does eventually notice, but by then the perpetrator is long gone.
Mobile robots exist that can find their way around independantly. Let's build some to roam the city, picking up all unattended objects, and throwing them into a built-in container. After a certain run, it should return home to empty the bin, and charge itself up.
Objects should be classified as either garbage or lost objects. Lost objects should have their owner determined, and be returned to their owner. Garbage should be further sorted as recyclable, burnable, or neither. Objects that are neither should be taken to landfill.
The robot should be strong enough to lift a tire, but not strong enough to lift a car. (Taking parked cars is asking for trouble.) The robot must be waterproof in case of puddles or ditches, able to move across grass, sand, and slopes up to 45 degrees, and able to rescue itself from falling into a ditch. If not rescue itself, then somehow call for human rescue. (Yes, I can imagine the robot using a cell-phone-like system to tell me in a synthesized voice that it has fallen, again, into a ditch, at such and such location, and would I please come rescue it. I can further imagine that this would be the twelth time this happened that day.)
Lastly, the robot must not wander into traffic. I doubt it would survive impact with a moving car. It must have some means of telling apart "road" from "sidewalk," and preferring "sidewalk." Bonus points if it can successfully taught to cross the street.