First, I need the standard disclaimer: My positing here is of a personal opinion and does not reflect the views of my agency. With that out of the way:
I'm a career National Park Service employee. I will absolutely encourage NPS-related positions for part of FIRE, and I've met many people over the years who have worked Park-related jobs into their retirement plans. This year is our centennial, so there's a huge amount going on right now, including the highest visitation in history for many Parks. My goal here is not to plug my blog, but we've got a couple articles that are relevant that I'm going to link rather than writing out info again.
There are a couple major paths.
1. NPS employee: Come work for the Service! A huge portion of the Park Service's staff are seasonal employees. If you're willing to jump through the hoops of becoming a Federal employee there are many fields you can work in. We've got an overview of the different NPS divisions on our (personal) blog about NPS life.
http://ourlivesaretheirvacations.blogspot.com/2014/05/more-than-rangers-organization-and-job.html . We've also got an (admittedly crappy) infographic about the odds of getting NPS jobs, because they can be INCREDIBLY competitive.
http://ourlivesaretheirvacations.blogspot.com/2016/05/the-odds-of-working-for-national-park.html2. Concessions employee: This is the formal term for all of the Hotel\Restaurant\Gift shop\Tour Guide\Etc. Positions run by private companies under government contract. These companies hire enormous amounts of seasonal staff to run their operations in the parks. The National Park Service has around 30,000 employees at the peak of the summer. The shared dorm\hotel style housing of younger people is a huge part of concessionaire hiring in many locations, although it seems like some of these companies are focusing on bringing in cheap labor from overseas these days. Fair insider knowledge: Some of these operations have very, very bad reputations at specific sites. Do your research, and talk to people who actually work there before signing up with any of them.
The big players nationally in this game are as follows: (In no particular order)
Aramark -
http://www.aramark.com/ Some Parks: Denali, Grand Canyon, Glacier Bay, Lake Mead, Mesa Verde, Yosemite, Olympic,
Forever Resorts -
http://www.foreverresorts.com/ Some Parks: Badlands, Big Bend, Bryce, Grand Canyon, Lake Mead, Mammoth Cave, Rocky Mountain.
Delaware North -
https://www.delawarenorth.com/ Some Parks: Grand Canyon, Yellowstone, Shenandoah, Olympic,
Xanterra -
http://www.xanterra.com/ Some Parks: Yellowstone, Grand Canyon, Zion, Crater Lake, Glacier, Death Valley, Mount Rushmore, Rocky Mountain.
3. Volunteers-In-Parks (AKA VIPs): The Park Service runs on volunteers. We couldn't do it without them. We average over 250,000 volunteers per year, overshadowing our official staff 10 to 1. Whether you have a day, a month, or an entire FIREd life, we'll happily find ways for you to help. We love putting people to work in their fields of expertise, and we have everyone from farmers to CEOs volunteering their time. We've got a little blog blip here:
http://ourlivesaretheirvacations.blogspot.com/2015/12/vip-becoming-part-of-national-park.html but the NPS Volunteers-In-Parks site is a much better source of information:
https://www.nps.gov/getinvolved/volunteer.htm. They've also got a good video (Although the first audio bite is a little rough) here:
https://www.nps.gov/media/video/view.htm?id=62147905-1DD8-B71B-0B01D39DAB6AF5404. "Other": This is the catch-all for the million-and-one things that exist around the parks. Every site is different but this could include things like flying helicopters for private tour companies (sometimes a park concession, sometimes not) or working in the hotels in the surrounding areas. NPS sites tend to have huge economic impacts on the surrounding communities, so there are generally a lot of jobs around parks that are related to tourism, but not directly tied to the parks. There are entire towns that exist as gateways to the National Parks, full of jobs (and often small-town politics) centered on the parks. Big examples are Tusayan, AZ (Grand Canyon), Gardiner, MT (Yellowstone), West Yellowstone, MT (Yellowstone), Mariposa, CA (Yosemite), or Gatlinburg, TN (Great Smoky Mountains).
If anyone has questions feel free to post them. As both a dedicated "Parkie" and a FIRE geek I think there are some good opportunities here for NPS to get some very smart people interested and for Mustachians to strategize on having one of the better jobs on the planet.