Sunseeker is a handy app
Shows sun rise and set times, sun rise and set locations etc.
Snapseed is a fun little editor.
Although I haven't used it in ages, Filterstorm pro is another great editor, almost a mini Lightroom (full disclosure, I was part of the Beta Test when it was first released, and I suggested a few features)
Strobox is great for doing lighting diagrams.
Voice recorder and notes for notes, peoples names, anything you need to remember.
Scanner apps (we use one on an IPad at work for listening to the fire brigade, and I use one for listening to aircraft movements)
You can also load a folio into it ! Handy to show your work off to prospective clients