Booking shore excursions on the ship?

[EWL]

Hi, just curious what kind of luck (if any) people have had trying to book shore excursions on the ship instead of in advance. We leave Saturday but haven't reserved anything partly because we like to do our own thing, but there are a couple that we might consider doing if there was space left. Is everything typically booked up in advance or is there sometimes space left?

Thanks!