You can use many roads to go from Montjuic (Olympic park on the hill near the sea) to the sea or to the city, even through cementery. The most popular seems to be through the tunnel.
And there is a cable car to go back up.
For longer downhill you can go from Tibidabo. You can use funicular to go up.
From Montjuic to Zona Franca
Through the tunnel.
Tunnel is the most popular.
Lots of skating in the night.
And this is some longer downhill from Tibidabo(hill above the city) to Barcelona (not on the maps obove).