A real good challenge is to mentor a high school robotics team. Check out First 

https://www.firstinspires.org/

Create a poor man's seismometer to register earth waves

Converting any "sense" no other "sense". For example - accelerometer data to sound, light color to sound, sound to movement, etc.

Make a number pad that displays the output of the number based on which numkey you input into the Arduino.

I have a HW project  and would be willing to chip in the HW cost if you complete it.  I can envision one of two options depending on your interests.  1) FPGA with simple addressable register architecture to convert 3.3v Teensy 4.0 IO to/from 5v TTL with no bidirectional buses between the Teensy 4.0 and the 5v TTL logic.  I would supply the schematic.  2) 4 layer PCB layout in KiCad, again I would supply a schematic using both surface mount and through hole parts.  This would require significant signal integrity experience.

Ah, OK. I have nothing in that area, unless you count hooking up a miniature air pump to a set of party blowouts.

If you have any interest in sewables (like FLORA or Lilypad), I have something: I'd like to be able to sew a display screen into a project.

While there are many possible solutions, my specific goal is for a 20x4 LCD display (with RGB background; Adafruit sells one of these) in the position of a wristwatch, taking signals from a nearby processor unit. It's for a major project I have planned with multiple "segments," so it'll be a good while before I dive into it, but it's the one thing for the wearable segment that I haven't quite figured out yet, at least to my satisfaction.

thanks for the replay. but I am more interested in something mechanical that would solve a problem