Just get the 60 day tourist visa online before you go, it’s £30 and if you have a little computer knowledge it’s very easy to apply, take around 5 days to comeback via email.
You won’t need a licence to rent one on koh Larn but it’s not a place for the inexperienced, steep hills with switchbacks on various road/track types on poorly maintained bikes with flintstones brakes.
When I flew to Vietnam the airline wanted to see a ticket out of Vietnam but when I flew back to Thailand they never asked but that could be just luck of the draw