A 556 ic with both 555s in astable mode. The first can be toggled between a timer for my physical therapy exercises with set timings of ~15s and a metronome with variable timings between ~0.3s (~200 bpm) and ~1.8s (~33 bpm). In both modes each pulse is accompanied by a flashing LED and a ~250 Hz tone (volume adjustable).

I needed a timer for a variety of physical therapy and metabolic conditioning exercises that all had timings that were multiples of 15s. I did first check to see if there was an app for that, but they were all so saturated with advertisements they were barely usable.

Sometimes a 555 (or two) is really all you need.

I added the metronome because, you know, why not? =)