Is there anything left undecided at the end of the novel?

As the previous answer states, several important things are resolved at the end of the novel: Jim is free, and Huck has been freed from the baleful influence of his "pap" who, Jim finally reveals, was drowned. Tom has recovered from the bullet wound he took during their recent adventures, and things appear to have wound up quite nicely. Huck remarks that

... there ain't nothing more to write about, and I am rotten glad of it, because if I'd a knowed what trouble it was to make a book I wouldn't a tackled it and ain't a'going to no more.

However, there is certainly a note of irresolution at the end of the book, with Huck planning to "light out" for the Ingean Territory in order to escape the civilizing clutches of Aunt Sally. We feel that Huck's fate does remain undecided, and that he will try to get away once again. However, judging from the above quote, whatever his further adventures might be, we won't necessarily read about them - at least not from his own pen, as he has so emphatically declared he'll never write a book again.