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Ordered by level of difficulty and probably age or experience of student, beginning with the easiest.
Time: 5-15 minutes. Plan: Printable
Excellent introduction. You start "turtle style" just instructing a pen to draw a square. Then if the student understands it and wants to go on, they can expand the code to take arrow key inputs.
Time: 20 minutes. Plan: Video
This is an excellent introduction because each bit of functionality you add has a visible effect. And things seem to "work" at each step... so that leaves it kind of open-ended; you just go as far as you can with the student.
Time: 10-20 minutes. Plan: Printable
This one has one of the best PRINTABLE lesson plans I've found.
Time: 10-35 minutes. Plan: Powerpoint
This is another one that feels like you've done something functional at every step. Plan includes valuable lessons like - first you put in "move -1 y" over and over, for the gravity of the ball - then teach the student the value of loops, by changing that to "move -1 forever".
Time: 20-35 minutes. Plan A: Printable. Plan B: Video - with slightly simpler final code.
Kids love games. Very simple rules for this car racing game make it feasible. Also note that there is a part 2 video / lesson, so if somebody really enjoyed it and wanted to continue on, they could.
Time: 15 minutes. Plan: Printable
This one is fairly simple in principle but does use a few variables. I figured that little bit of abstraction makes it more advanced, but it should be very easy for a student that seems experienced.
See the rest of the lesson plans here, including Helicopter, making a Tank, Asteroids, and Space Invaders: http://www.eastonhome.co.uk/Scratch/index.htm