Measure the speed of a stationary bike with belt drive

15 km/h is 4.2 m/s, then a distance of 40 cm will take minimum 100 ms.

Place two LED/phototransistor combinations 40 cm apart, with the boxes passing between them. You can place both LEDs and phototransistors in a tube to make sure the phototransistors aren't disturbed by other light sources. You can use a modulated source signal for better noise immunity.

When the microcontroller detects an interruption of the signal (box passing) it just has to look for the same interruption from the second detector minimum 100 ms later. Convert time difference to speed.

Note that photo-interrupters like this are used everywhere in logistics and manufacturing, and an industrial strength device may give you a higher reliability, though that comes at a price.

This one has transmitter and receiver in the same device, and uses the reflector to bounce back the signal.

Alternatively, maybe you van work with the voltage controlling the belts motor. Like when it's a variable speed synchronous motor.

The way to do this is with a Hall effect sensor. I like the Allegro A1324.

It's a simple device. Give it 5v and 0v on pins 1 and 2. Pin 3 gives a voltage proportional to the magnetic field. You can sample this with an ADC, or you can use a comparator to convert this analogue value into a digital square wave.