As A Week in the Woods now stands, it is an optimistic and encouraging coming of age story about a boy who finds a new environment makes him a better, freer person who is more on his way to becoming a man. There is significant foreshadowing of Mr. Maxwell's misadventure and the ending in the form of the question asked in the opening paragraph of the story, and the ensuing story pursues the answer to the question asked:
[At] quarter of seven on a Friday morning ... a question formed in [Mr. Maxwell's] mind: Why on earth do I do this year after year?
To rewrite the ending poses a difficult task because most choices will change the genre of optimistic coming of age to something else. If the foreshadowing is going to be honored in the rewrite, some sort of doom must befall Mr. Maxwell. If the genre is to be respected, Mark must have an epiphany and become a better person. How many different endings can do all this?
What are some options? You might make it into a postmodernist tragedy and have Mr. Maxwell die thus changing Mark through loss and grief, answering the question with the theme of sacrifice.
You might make it a romance and introduce a girl who is in danger whom Maxwell and Mark join forces to save and with whom Mark then falls in love, answering the question with the themes of cooperation and love. This romance might still fit the genre of coming of age if falling in love makes Mark a better person with greater mature qualities.
You might continue with coming of age and have Maxwell drive off with a falsely accused Mark only to find that Jason is running frantically behind the car to confess all. Here Maxwell would be humbled and chastised by circumstances while both boys have celebratory epiphanies, answering the question with the themes of humility and truthfulness. This might be good except that (1) Mark runs the chance of becoming arrogant and bitter because of his triumph over Maxwell and (2) Jason may seem more heroic than the hero.
These are some of the options for rewriting the ending, and these are some of the problems you must overcome to rewrite a closely constructed story ending where one thing leads to another: foreshadowing leads to future events, characterizations lead to actions and trait development, plot twists lead to later events.