This is a good paper describing the use and characteristics of rich problems.
http://math.sfsu.edu/hsu/papers/HsuKyshResek-RichProblems.pdf
(The teaching method here is Teaching With Tasks.)
Has 3 examples of rich problems
- First is rich and fairly basic and doable.
- Second is excellent. #differenceofsquares (I like this one the best. 😀)
- Third is quite open-ended.
The article does a nice job of showing how instruction and (formative) assessment go hand-in-hand.
Finally, and perhaps most importantly, it has good mathematics.
