I’ve been listening to a lot of audiobooks as I walk around the city with our newborn baby. Here’s my “highlight” trick: after pressing play in Audible, I switch to a text editor app (I use IA Writer.) When I hear something in the book that I want to make note of, I press the voice dictation / microphone key on the keyboard. This pauses the audio while I dictate the sentence into the phone. When I’m finished the dictation, the audiobook resumes.

Not as convenient as highlighted notes in Kindle or Medium, and socially awkward when there are other people around, but it works to help me collect a book's essential ideas.