I have used my credits so many ways, usually in the spa, because when I board, I learned years ago to get up there and just book right away, as I don't do spa at home. They have a calendar telling you what are port and sea days.
Sometimes at the end, I've spent them on crazy expensive creams from the gift shop from the Esteee Lauder section with products I know but wouldn't buy.
At the end, wine at dinners (I'm happy with pouring wines) and small gifts. I never leave a penny behind! Never knew I had a choice to contribute to crew welfare fund, usually do that by just getting a cash out charged to my account. Have asked before and was told not with sbc's. I supposed it was where they came from, and whether transferable to cash.
I don't like 9.5 hour tours anymore paid or fee, but there was a day when I'd gladly take them on.
Right now I am at Disney World with my son, DIL and grands, we just sat and talked for hours. I would LOVE a credit for a massage, they are running me ragged!