Balloons: some coloured balloons float from the bottom to the top.
If they reach the top they disappear and remerge after a second
or two with a small random change in the X position.
If the user clicks on a balloon it pops and the score increases.
The balloons rise at a different rate for each colour - the faster
the rise the higher the score.

I have this one playing on the white board when the children
enter the class. As expect one will touch a balloon which pops.
Yes they all want ago so herding them back to their seats is required.   
I then ask them what is happening? They note there are balloons
that rise and with a bit of prompting they will discover about the
colour-speed and scoring. There’re begging to analysis the problem
thinking of routes to obtain the goal of the programme.