Cinema Roulette
Go to a cinema but instead of watching the movie you bought the tickets for, walk into a random screening room and watch whatever movie is shown there.
Don't spoil yourself
I highly recommend not checking out which movies are currently in cinemas.Book a ticket
If you book a ticket online, I suggest covering the part of the screen, that shows images and titles and only checking out the starting time. Book a ticket for the first movie at that pops up at your preferred time. When done skilfully, you'll only know the title of that one movie, which you won't be watching anyways. Should you have already watched a movie recently, just pick that one.
If you book a ticket directly at the cinema, ask for the next movie possible and stop them, when they try to tell you titles of any other movies.
Pick a screening room
Take a look at other people in the cinema. Find some that look like they might have a good taste in movies and follow them into a screening room. I suggest picking unpopular seats first, and then changing, when you feel like everyone else got their seat.