In Royal garden of Prague castle takes places this weekend the Beer Festival with more than 70 mini Czech mini-breweries.
The fee is 475 CZK if you buy the ticket ahead of the time, 500 CZK when buying the ticket on the spot. Once being inside, you have unlimited consumption of the beer included the ticket.
Even more - you can buy some snacks, but it´s absolutely ok to bring your own food and water, sit down on the lawn (which is usually prohibited as you are in Royal garden) and have your own picnic.
Opened: Fri - Sat. June 15 - 16, 2018, every day 2:00 PM - 8:00 PM.
More info here