While I have never been a volunteer for the NPS, I used to work for them (ok I volunteered time but was compensated better than volunteers). The volunteers I used to manage/work with were some of the best people I have known and I still stay in touch with many of them.
Some volunteers only stay 4-6 weeks (sometimes volunteering in 2-3 parks a summer) while others come back season after season.
Each park is run differently but there are normally several basic positions at every park.
- Campground Host: Can be adult babysitting, but is a great way to meet lots of new people from all over the world. Helps with keeping campground clean and things orderly. With NPS if something gets out of hand there are usually LE Rangers nearby to handle it.
- Meadow Rover/Trail Patrol/Trail Head monitor: Hike around or stay in one designated place and answer questions from the general public. Discourage unprepared people from doing stupid things. Also during SARs (Search and Rescues) monitors trail heads & junctions and such.
- Trail maintenance: Normally through an outside non-profit.
- Visitor Center: These can be with the NPS or the park affiliated non-profit. Answers questions sells things at the non-profit's store etc.
- Biology/ecology samplingg: the scientists often have volunteers to help them gather data, cameras, plants, etc. These can be hard to find but they do exist.
In addition to those positions you may be able to do a "Ranger talk" in the campground amphitheater. With budget cuts and under staffing most parks are cutting back on ranger talks(I will save my rant for another time). My volunteers were key to making bi-weekly ranger talks happen.
SAR. If you are willing you can always help out and due to park policy you are actually paid for hours worked on the mission. This could include hiking trails, shuttling people, staff command post, etc. It doesn't have to be physical.
Each park has their own unique opportunities depending
Long story short the NPS would grind to a halt without volunteers. When I retire I will volunteer for the fun of it. Some parks are great to volunteer at and unfortunately some parks are not depending on the administration. Some have housing available others may just have full hook up sites. Normally you will get a free place to stay and a small stipend for food.
Don't forget to check out the affiliated non-profits working in the parks.