Something I find really annoying to do is filling up a jug of water. Because the water dispenser lets out the water quite slowly, it takes ages to fill up the jug completely. So I came up with the idea of making a water sensor that will let me know when the jug is full, so I don't constantly have to focus on it.
Because I didn't have very much electronic experience, I made it from trash. To make it I used a cheap kid's toy that I got from a kid's magazine. Basically, it was a small sack that started laughing if you squeezed it. The sound it makes is not perfect, but it works fine. Instead of this any toy that makes a sound when a button is pressed could be used. Happy Meal toys would probably work perfectly.
The two open ended wires are just regular uninsulated 1.5mm wire. I attached it to the toy with some hot-glue (no soldering).
Making this took me just about five minutes. The rewiring was very straightforward, and thankfully there was no soldering needed.
How to make it
Here's a video of the sensor in action: