There are a whole host of things that OBC can be used for other than shore excursions. Both pre cruise and during, salon, spa, on board shops, extra gratuities, specialty dining, casino and many others.
And the post I referred to also included several very good suggestions for the OP as well. What is wrong with doing some research then formulating questions to pose on here.
If you can do it for $50 just do it already, 4 ppl in a standard closet er I mean cabin would be ridiculous. We can’t make the decision for you. Even if more than $50 I’d def pony up, though I’d of not put myself in that position to begin with.
Why does doing one’s best have to involve a prize?! Can’t just doing your best and knowing how hard you worked for a good outcome be “prize” very good lesson IMO. Enough real problems in the world already than to get riled up over a t-shirt.