Electronic – Detect aluminum using an inductive sensor

I would be thinking in terms of running a grid of some very fine wires stretched tightly across the inside of the outer glass — single strands of AWG40 spaced every 3-4 inches, horizontally and vertically, would be nearly invisible.

Scan the wires individually for increased capacitance; if the wires that cross in your "target area" show increased capacitance at the same time, trigger your event. Such a system would allow the creation of multiple "hot spots" of various sizes, each triggering its own event.

You can always use IR sensor. But they need to be cleaned. Rather if you want you can use ultrasonic distance measurement sensors. This way you can calculate the height of the trash in the bin when placed under the sensor. And you don't need to clean the sensor. Another advantage will be that no matter what container you use, the system can be used. This saves your thought on mounting the sensor inside or along with the bin and the bin remains replaceable. Programming and getting results from the sensor however depend on the thoughtfulness of the programmer.