I just call out 'Hola most of the time .
Seems to be generally reciprocated, if not in fact more often initiated by the othe party
.
When I was cycling on the North coast many would also wish me "Buen Camino" because that's what they assumed I was doing..
I've found cyclists, and drivers, and just
people in general more open, friendly, and spontaneously hospitable in Spain, than the UK.
Not that the majority in UK are hostile (you tend to get back what you give out)
It's just in Spain, the cyclist seems to be welcomed, and even celebrated - especially when going up hills fully loaded
Lots of encouragement - rather than seeming to be annoyed by the temporary inconvenience.