Get a summer job or internship with another agency or organization and build from there.
NPS isn't so great, they are a bunch of bunny huggers and rock preservationists.
Try Forest Service, BLM, or DoD - anyone that has a lot of land. Or try state agencies (Department of Natural Resources or Dept of Envirmental Conservation and such).
Or you have groups like The Nature Conservancy.
One your best bets might be getting on with a university as an hourly. You just have to find the right people, and that can be a lead in to graduate school.