Another vote for Organic Maps. I use it as a lightweight maps app for backcountry or traveling in foreign countries where I don’t have a sim card. You can also record tracks in the app, or import .gpx files. In airplane mode it has low impact on battery consumption.

I was also pleasantly surprised to find out iOS Star Chart app (https://apps.apple.com/us/app/star-chart/id345542655) functions entirely offline. Recently used it while camping, and it just needed a GPS coordinates fix to adjust sky map to location.