Amusement parks employee people from all different fields: marketing, accounting, advertising, landscaping, catering, management, tourism, etc... If you're just interested in being involved with a park as opposed to the coasters themselves. For coasters specifically, there's mechanics/maintenance, civil engineering, maybe some other form of engineering (I'm not too sure about that one).
-------------
Po!nt of View: A different look at Roller Coasters.
http://www.crosswinds.net/~justmayntz/thrills/