Creating an interesting assessment task that gives the teacher a wealth of information about students is actually very easy.
For example. take some standard (boring) area questions - find the area of a rectangle with length 12 cm and width 4 cm; find the area of a triangle with base 20 cm and perpendicular height 12 cm, etc.
These questions can be easily twisted into an open-ended problem that allows for a range of responses :- "Find as many different shapes as you can which have an area of 48 square meters. You must justify each of your answers."
You instill in the kids that it is important that when doing a RAT, they must give the very best answer that they can.
The responses will include simple rectangles from the weakest students, triangles from those with a bit more knowledge, up to tricky compound shapes from very good students. The very best may come up with a circle that has an area very close to 48 m^2.
Here is another interesting problem that involves areas of squares and circles, although at a higher level of abstraction than the previous question :- "Which fits better, a square peg in a round hole, or a round peg in a square hole? Justify your answer mathematically."
Now you try one. Take this boring question and make it into a RAT. And please post your question to the list.
Find the mean of the following set of numbers: 2, 5, 4, 7, 8, 2, 4, 3, 1, 0.