The OP was titled 'Useful uncommon gear'. You have received a number of very useful and practical replies, so I will pick up on the 'uncommon', perhaps less 'obvious', part. You may find that having pockets available, beyond those on your BCD, is particularly useful. Many (and, not just newer) divers struggle with where to put their 'stuff' - lights, signaling devices, slates, folding snorkles, etc., and the pockets on some BCDs are simply not very accessible, particularly on weight-intergrated units. So, items like X-Shorts - shorts with pockets that can be worn over a wetsuit - or some of the commercially availble pockets that can be attached to a waist strap or BCD, might be examples of uncommon, but very useful, gear.What besides the obvious is in your bag on practically every trip for pretty much any dive?