This is one of my favorite gardens in Barcelona. I take the subway to Mundet, then I walk uphill living the buzz of the city behind me. On Wednesdays, the entrance is for free (the rest of the days you need to be a small fee to go in). The maze isn't very big, just enough to get a bit disoriented and have fun trying to find the exit, but you want to be spending the night inside (this is not a Harry Potters movie!).
After walking around the labyrinth, make sure to keep exploring the rest of the park: there is a pond, a water stream with a bridge, and a hidden garden in a narrow valley.
Sometimes I've taken clients here on garden tours, and they were delighted! It's a secret gem.