I've also seen "en sandstrand", but only in an older book. Is "strand" preferred over "sandstrand"?
In general, yes. I'd use sandstrand only if I wanted to be explicit about the beach being sandy.
So presumably stenstrand for a pebble beach?
'strand' is a commonly used English word for 'beach'. It should be accepted
"The pirates are stranded on a beach," hope that helps with remembering this. :p