The agency Graphic Detail were re-building a website for Avanti Finance, using WordPress. They asked me to create a loan calculator as a WordPress plugin. It would site front and centre on the Home Page. No pressure then.
The loan calculator offered you several choices by sliders and check boxes: how much you would like to borrow, or how much to repay. And how frequently you would repay. There is also a choice for interest rate, so that you can get a feel of how future rates may affect your ability to repay.
There was only a small bit of PHP involved, letting the client configure messages and almost every moveable part of the behaviour.
All the parts synchronise while you change them, which needed me to develop a delicate network of JQuery dependencies. At times it felt like tapping my head plus stroking my tummy counter-clockwise.