Each year I ask the students what they would like to "make" and what problems they would like to solve.
Slow feed hay bot
Deer fence
S.M.A.R.T alarm clock (link)
Something that can fly or go in the water
Soda bottle rocket / Water Rocket (link)
Brushbots (link)
Rain Alarm (link)
Electric generator
Levitating water strobe light
Simple electric motor
Lights that change color and patterns to music
Backup phone charger
Light that turns on when you are near it
Night vision goggles
A better backpack
Hovercraft
Drone
Water wheel generator
Rubberband airplane