NPS Emplopyee here.
You might try Avenza Maps. It's free for personal/recreational use. In addition to downloading offline-ready maps, it also allows you to collect basic GPS points and import a limited number of your own maps for use as well. From within the app, you can download a number of free maps including all of the USGS Topo quads and National Park maps from their "Avenza Map Store." (It's a "store" because users can upload their own custom maps and make then available for a small fee.)