When I was a teacher at a tech level school I once promoted a competition where the students where given a circuit on veroboard and the smallest functional board won the competition. Prize was a box of beers
But that is too simple for your case I guess.
On a previous year the competition was to build a miniature crane; the concepts required where the subjects of consecutive classes (e.g. simple motor control, PWM, etc.). The competition involved lifting 3 different weights onto 3 miniature "buildings" I made in Styrofoam, 3 different heights.
Again maybe too simple and similar to the "drone / robot" things you mentioned.
Last year I taught at that school the competition was to build a toy car controlled in 2 different fashions: sound (e.g. clap) and light (point a light to it and it ran); when light controlled it would run for as long as sufficient light hit the sensor, when sound controlled it would run for a specified time and then require another clap to go again.
Once more, it was designed to use different concepts learned over the course (e.g. use of simple light sensors such as LDR's, use transistors as switches, use of comparators, simple 555 timing circuits, etc.)
Also too simple I would guess...
As said, these were all for a technician level school so perhaps too simple. But maybe you can use as inspiration.
I had a professor that liked running simple competitions and they were loads of fun. One of the classes he thought we had to build a FM receiver; the team's radio where he could listen to the most different stations won. I remember we used some Phillips IC and build a board around it, in theory simple but to make it work sufficiently well (i.e. get all inductors and capacitors right, etc.) was quite challenging and fun. We won the competition
The same professor also taught a basic DSP class. The competition was like this: we were investigators in a company that had found that a group of employees was leaking industrial secrets through a custom build communication device; we had found though that we could "listen" to the signal when tunned to a certain FM radio station due to interference. Our job was therefore to, given a sample of the audio signal where the interference was audible, analyze it and crack the encoding, and then decode the message.
We did it in Matlab. On the day we presented the findings the teacher would then run a new, not seen before sample through our code and see if could decode this sample as well.
Lots of fun - but this in our case was done purely on software (Matlab). Maybe you could have the students building some hardware to capture an audio sample, process it in a DSP / dev kit board, and show the decoded message on a display or something?