On board credit can be spent on anything on board-drinks, shore excursions, tips, speciality restaurants, t-shirts, spa appointments, jewellery........basically any thing that costs money.
The godfather of all cruise sites is boards.cruisecritic.com - select your line and read away for all information you will ever need.
ps: it is pretty difficult to fall over so don't be worried about your wife upping your life insurance just before you go!