Music (iPod) & Headphones - Music = necessity. Music helps so much at the gym whether it's to help get you through the workout or to avoid talking to people and getting distracted. There's nothing a good sing-a-long can't help. That includes exercising.  - Apple.

Water Bottle - Water is SO important. Keeping hydrated helps with everything; exercise, cognition, digestion etc. I can't go anywhere without my bottle. This comes in so handy, I fill it up as much as possible. Definitely a bag essential. - Paperchase. 

Tissues - Incase there's no loo roll, or you get a sniffly nose (I always seem to when I walk to the gym and back) or you just need to give something a clean. These little packs are so useful. I guarantee you won't regret throwing a pack in your gym bag. 

Perfume  - A must-have after a sweaty workout to make me smell nice again. Especially if I'm attending something social afterwards. I carry around a cheap scent just for a top-up when I need one. Carrying around an inexpensive perfeume also means i'm not upset if I do lose it or break it (I'm clumsy!).  - Next

Deodorant Wipes - These are such a life saver if you have to rush somewhere straight after working out. If you're forgetful like me, you might go for a run and realise you have 15minutes to be at a meeting. LIFE. SAVERS. These do come in so handy for them sweaty post-workout moments  - Primark.

Deodorant - For the same reason as above I always carry around a roll on deodorant with me. This is for those days where I go straight to meetings or lectures. No one wants to sit next to a stinky person. I just find it is easier to keep one in my bag so I'm never without it. - Faith in Nature Roll-on Deodorant.

Hair ties  - A female necessity! Always carry spares.  

Anti-bacterial - Definitely one to carry around. Gyms/swimming pools are breeding grounds for illnesses and bacteria. Yuck. So many germs. I carry this around to use before and after to at least help try and battle some of that bacteria and keep my hands clean. Also helpful if the bathrooms run out of hand soap or something.  - Refan, Bulgaria. 

Towel - Obviously something I need after swimming to get dry but, I leave this in my bag just in case I ever need it. You can use this as a 'sweat towel' in the gym or for washing afterwards.

Carrier Bag - This one is for smelly or wet clothes post-workout/swim. You don't want the clothes loose in your bag with the other things you're carrying around. 

Spare Undies/Socks - I always, always, always carry around spare socks. And now a spare pair of undies for after my swim. That's those primary school memories of having to go commando post-swim coming back to me. This just makes life simpler having them there as a backup. Most the time I don't actually end up needing them but sometimes, they turn out to be an absolute godsend (especially those socks if you're going somewhere post-workout - uck, sweaty feet are gross).

Talcum/Baby Powder - I use this when i get changed after every workout/swim. It really helps absorb any moisture so you don't have that damp, sweaty feeling. It helps to prevent nasty rashes or athletes foot. I always sprinkle some into my socks/shoes and use some on my towel to help me dry. It isn't just for babies, I swear! - Johnsons Baby Powder.