In the morning I went to the top of Tibidabo Mountain by public transportation and then hiked all the way to Sant Cugat de Valles which is about 7 miles / 11 kilometers.
The views of Barcelona from the amusement park and church at the top of Tibidabo are fantastic and shouldn't be missed.
The hike starts at the end of the road that goes behind the church to a parking lot. For easy phone navigation, use the Google Maps or Wikiloc links on this page:
Hike from Tibidabo to Sant Medir and Sant Cugat.