I got a couple of apps I'd recommend in a heartbeat.
Spectdroid is a spectrogram app. Its unreasonable how often I'm using this app. I got some mild tinnitus that comes and goes and this app allows me to find out if I got some actual weird buzzing I'm the house or if it's just in my head.
And LocalSend is an amazing app for sending files between various devices and OSes over a local network. I no longer need to set up file shares, plug in my phone to a computer, or use cloud storage just to transfer over some files.