mmm what an incredible question. I am not sure there is any answer that could satisfy what you are asking, actually. I work in mixed abilities classrooms, and that is seriously challenging to get right sometimes.
The first place to start for me is "what am I actually assessing?" I want to avoid the traps around "follow me" type courses, so I try to throw in a lot of research questions. Is this testing their research skills? Or their ability to use Google? With assignments that do not involve fluency quizzes, I try to ask a variety of questions to allow the not so clever students to be successful, that is biographies, how to's, what happens and so on, all the way up to what do you think of..., or, given this, how does it impact on that... for the brighter kids. That actually works by the way, the brighter kids get bored with topics they are not challenged in so do actually pick tougher questions.
Fluency quizzes should always ask questions as afterthoughts. For example, "Does water have a polar ionic bond or a polar covalent bond?" A simple three word answer, but that is followed by "Explain your reasons for suggesting this." Many students stumble around these questions, even if they get the first part right. They will often say something like, "Water has a polar covalent bond because hydrogen has one electron and the oxygen takes it." This shows, for me anyway, they get some things about the concepts of covalent bonds and have been able to piece enough together to connect hydrogen and oxygen as non-metals, but do not get the idea that hydrogen requires a polar covalent bond and will not bond with any other atom that cannot provide it. The "explain your reasoning" is a pretty powerful way of getting kids to talk about their learning and that is not always successful. Of course, I have often been quite open with that question too.
A lot of students have been struggling with the essential language of science, so I might start a topic with building a glossary. Pick 5 words from this list and find out what they mean, add them to the glossary. They copy and paste, of course, but then the next assignment is "What 5 words did you find definitions for? Did you know what they were before adding them to the glossary? What do you understand two of these words mean since you now know the definitions of these words." First, the words they picked, second a simple yes no type question followed by the one that makes them think. "What do you understand..." can be a scary question for students, and I have had responses ranging from "not a lot" to some detailed and precise theory on the topic. This works well on science lab work, if you toss in "how can you make this more fun?".
This sort of thing lends itself better to some topics in Maths than it does in others, Statistics as opposed to Algebra I am thinking here. (A lot of my teaching has been across a range of subjects, from English Lit to History to Maths and Science, you have to when plugging holes. I am not doing any IT, Humanities or Social Sciences this year, it's all Maths and Science, and I am not getting bored.. )
Don't know if that helps or not, but it helps me clarify some of the things I have been doing so I can explain it to parents better.