Fringe Festival is a festival I've heard of for a couple of years now. It's the largest art festival in the world and takes place in Edinburgh. The festival lasts almost a month (3rd-27th of August) and takes place throughout the whole city of Edinburgh. There is no 'entrance fee' to visiting the festival, but there is a fee for all shows. You can buy separate tickets to any show you want to visit so you can truly make it your own experience. There are shows ranging from ballet to theatre to comedy to magic. Everyone can find their cup of tea. 

Make sure you book in advance, however, since accommodation can become expensive as well as flight tickets. Learn more about the festival here: https://tickets.edfringe.com/