[jmkb]

Use an app that permanently downloads maps -- I like Mapy.cz and Organic Maps. And bring a waterproof phone case, an external battery, and a charging cable.

And also bring a waterproof paper map because it's lighter, easier to read, more resilient, and more fun.

[nicbou]

OsmAnd is the pocket knife of cartography. It's by far the most flexible map I've use, although it's not as user-friendly as others. You can really choose what gets rendered and how.

[petodo]

+1 for mapy.cz for hikers, they use OSMaps, but have definitely more hiking trails than osmand or maps.me and better readable