We've always used our OBC for Cruise Next. I'm assuming that it is not refundable -- it is from NCL. We've had $600 in OBC for our last five or so bookings. It just magically appears when we book a cruise. That's often what we end up spending at least some of it on -- it depends how much we spend in the spa.
To the OP -- spa is a good way to spend a chunk of change. Another spend is a picture package. We did this once and truthfully got some great photos from it. You can also spend little bits of it by purchasing the a la carte food/drinks (Starbucks, ice cream, etc.).