Diving! Course run down in Plymouth, next to no cost, takes about a week and if you're careful you can do the initial course at the start of your hold and get up to Sports Diver (the next level) before you finish your hold! There are always expeds going on, not that you'll be able to go on them if you reach the front line as aircrew! You'll even be able to caving in Mexico, as per Dan's suggestion, to rescue some to rescue some terrible visa breaking, uranium stealing, bearded servicemen! You also get a better tan at sea which is a must as aircrew!
