I agree with Julia.
на can have different meanings in both the Prep and Acc cases. Here are some of its meanings in both:
"on" (literally) + Prep - Мы сидели на стене и болтали.
"in" + Prep (a synonym for в, when you have to use на not в) - Мы были на работе и на уроке, хотя мы не смотрели матч на стадионе. (likewise, points of the compass take на -
на севере, востоке, юге, западе)
"to" + Acc (a synonym for в, when you have to use на not в) - Мы ездим на работу и на Кавказ и на почту etc...
"by" + Acc (when showing a margin of difference, with a comparative construction) - я старше его на пять лет, и она моложе меня на неделю.
"for" + Acc (when showing a planned activity) - я опоздал на лекцию, на самолёт... я экономлю деньги на каникулы (because something will happen then)
it's also used in slang to mean something like "here you go" when passing s.one s.th, although I think the Nominative comes after (!?) - вот тебе и на! (meaning something like "well there you go!") на деньги! (here's your money!)