What did McCandless do when he returned to the U.S.?
This question is potentially huge, because I believe that you are referring to McCandless's return to the United States after paddling his canoe into Mexico. The reason the answer could be huge is because that event happened early in McCandless's wanderings. I would like to limit my answer to what McCandless did soon after coming back to the United States.
For the first night back in the United States (January 18), McCandless spent the night in jail for attempting to re-enter the country without an ID. During the next six weeks, McCandless wandered aimlessly. He went as far east as Houston and as far west as the Pacific Ocean. His journal states that he got a job and an ID in Los Angeles sometime during late January or early February. By February 9, McCandless was camping at the bottom of the Grand Canyon. Then, on February 27, McCandless arrived in Las Vegas. He lived with other vagrants in Las Vegas until May 10. Then he left the city and went back to his wandering.
On May 10, itchy feet returned and Alex left his job in Vegas, retrieved his backpack, and hit the road again,. . .