There’s a bike rental place on paseig de peixina, valencia. Just googled it and valencia bikes came up, as above Dutch style city bikes, my mate hires mountain bikes from them so maybe it’s a neighbouring rental place as they aren’t on the website. I ve bought bike bits off mammoth bikes, maybe message them see if they can help.
We use the train from valencia to gandia, hop on and off and cycle back. Best routes are between sueca and valencia. Good beaches, albufera natural park with tarmac forest paths.
I ll be there in November if you want any more info just ask.