For Android:

I like "Maps Ruler" for tracing a route (like for a walk around a few blocks) and seeing how long it is. Also for figuring the distance between locations you can identify on a map. It supports satellite view too, so you can identify, say, two houses that you are familiar with and see how far they are apart. This would also be good if you lived on a large property and wanted to range various places from your house, so you could set your rifle scope's elevation correctly.

I like "What3Words" for a way to identify locations using words that are easier to remember and convey to others than GPS coordinates.

I like "SkyView" for identifying celestial objects in the sky overhead.