Add a coin slot to your Arcade Machine
This YouTube video tutorial is a little old (from 2014) and shows how to do it specifically for the Porta Pi games machine but you can you try this with any Raspberry Pi based arcade machine.
It runs through how to add a coin slot to your machine so that when you drop a coin into the slot it is recognised but the Raspberry Pi and adds credit to your games. It adds yet another level of authenticity to your Arcade.
The video is by Ryan Bates and runs through what compnents you need, how to install them and get it all working. A very good video to watch.