1. In chapter one, Huck dismisses the Bible because once he finds out that Moses has been dead "a considerable long time" he states, "I don't take no stock in dead people." Huck is completely serious; however, anyone else can see the humor in this statement. It is quite funny, and I'm sure Twain meant it to be.
2. In chapter 17, Huck is describing Emmeline Grangerford's pictures and states, "I didn't somehow seem to take to them, because if I ever was down a little they always give me the fantods...I reckoned with her disposition she was having a better time in the graveyard." Here Huck is trying to be respectful to the morbid pictures of the deceased Emmeline, but really struggles; they give him the creeps and he figures she's happier in the ground. As a kid, he's serious when he says these things, but this is a great chance for Twain to make fun of those ultra-serious poetic types that are so absurd.
3. In chapter 3, Huck is pondering everything that the widow and Miss Watson have told him about heaven, and figures, "there was two Providences, and a poor chap would stand considerable show with the widow's Providence, but if Miss Watson's got him there warn't no help for him any more." Here, Huck is seriously thinking that there are 2 types of heaven, a nice one and a mean one-based on the two versions from his care-takers. We can see how this is funny; Twain here is satirizing people and their varying opinions on heaven, and how God, according to people, has such polar opposite personalities.
Those are just a few examples, and I hope it helps!