This question is asking for level 2 (and up) type questions, as those types of questions force a reader to provide an opinion based answer supported by textual evidence. Your questions could stay fairly broad and focus on things like character analysis or thematic support, but your questions may certainly be more specific in nature as well.
For example, you could write a question that asks a reader what he/she thinks it says about Huck when he so willingly invites the duke and king along on the raft. You could also ask a broad question about Huck allowing the two unsavory characters to stay. Already by the end of chapter 19, Huck knows that the king and duke are lying con-men. Huck is certainly intelligent enough to know that these men are out for themselves. Support comes later when Huck and Jim are forced to sleep outside of the raft shelter, yet Huck still tolerates them rather than just leaving when the two men are not on the raft. Why would Huck act this way?
As chapters 20 and 21 show readers, the king and duke are hilarious characters. There is a lot of wonderful comic relief in these chapters, as the king and the duke appear to be bumbling idiots, yet they are incredibly cruel to Jim. Making the fake escaped slave leaflet is especially cruel and actually quite dangerous for Jim. Ask a question about whether or not a person believes that the two are as dumb as they seem.
Finally, Twain could have had the king run his first scam in a variety of ways, yet the first scam is held at a religious event. What might this say about the king's malice or the gullibility of people of faith?