My MC is seventeen, smart, sheltered, and has lived his entire life in Portland, Maine. His biggest forays outside of it are still in northern New England or nearby areas of Canada. There's a single exception, but he was accompanied by his then-girlfriend the whole time.
As the first leg of an impromptu road trip, he's ended up in Boston with a bunch of drunk guys he barely knows the names of and no idea what he's doing. I'm not entirely sure where in Boston he even is, because I don't know as much about the city as I do some others in the novel, and am not entirely sure where early-twenties college graduate trust fund party kids gravitate. Wherever it is, it's new to him.
What would he notice right off the bat? What would become obvious just slightly later? Racial diversity is a possible big one, but I'm not sure what else.