I've built thousands of zaps for people at this point, but I have fond memories of the first complex zap I built when I got into Zapier four years ago - and I think it's still very useful now.
I ran (and still run) a small business matching language teachers with students in their area. Our CRM was a Google Sheet, each match was a row, and each row has a status, and date on which I wanted to email the teacher asking for an update.
Realising that a Zapier will only find the first row matching a criteria, I built a zap that triggered with a webhook and ended with one... and triggered itself again. I had it find the first row matching the day's date, action on it, then change the date column to a new date, so that next the zap ran, it found the next row down.