You’re correct, you can’t use OBC to book excursions in advance but you can use it to pay for them once you’re on board. We’ve never used the casino so can’t help there, although I don’t think you can use OBC - I’m sure someone else can advise on that. All the other things you mention can be purchased with OBC, plus you can use it in the shops, including duty free. We normally use ours to pay for drinks, the occasional specialty meal and duty free cigarettes.
Hope this helps.