A hike from Estivella train station up to Mirador del Garbí, then to Sierra Castle and down to a bus stop in Serra.
For this hike you would need to use C5 (or C6 if you choose different starting time) train to get to Estivella and then Metrobus from Serra to Bétera and then metro from there.
The C5 train leaves from Valencia Nord train station at 7:57 in the morning and arrives at Estivella – Albalat dels Tarongers train station at 8:47.
Train schedule: http://www.renfe.com/viajeros/cercanias/valencia/index.html
The length of the route is 14 kilometers with about 600m of elevation gain. That can take you at least 3 hours at a fast pace, but most likely about 5 hours with some stops.
The Edetania 230 bus leaves Serra at 12:20 / 13:15 / 14:15 / 15:35 / 16:35 / 18:05 on weekdays, so you can choose what pace to walk with.
Bus schedule: http://www.edetaniabus.com/horarios.aspx
Then there’s metro from Bétera every 30 minutes.
Metro schedule: https://www.metrovalencia.es/horarios.php?page=143
Mirador del Garbí
Ermita del Garbí
Castillo de Serra
For now on Strava (with an option to download .gpx), will post on Wikiloc later.