Lock It or Lose It
The most important item to pack is a lock. If it’s a combination lock, commit the combination to memory. If your lock needs a key, keep your keys with you during your workout; hanging them on a wrist coil works well. If you plan on swimming, keys may be tricky to keep secure, unless the lifeguard can keep an eye on them.
Hydrate and Energize
A fresh water bottle needs to be part of your kit. If you’re planning a long workout, pack two of them to stay hydrated. An energy bar can prevent a post-training crash. Choose a bar that is low in sugar but high in vitamins and minerals so that you don’t undo all the good work you’ve done during your workout.
No Shirt, No Shoes, No Workout
An extra pair of gym shorts, a T-shirt, athletic shoes, socks and undergarments gear you up for spontaneous workouts. Add a bathing suit, goggles and a swim cap for the pool. If you play squash or another sport, you need a set of fresh balls, court shoes, goggles or safety glasses, and your racket, unless your club provides the latter. Weightlifting gloves will keep you from getting blisters and a yoga mat's always good for a nice stretch. Pack your earbuds if you plan to enjoy watching TV on the treadmill or need them for your gym's cardio theater. Take your smartphone too. Some machines have playlist, workout apps and heart monitors that you can hook into.
Personal Hygiene, Keep it Clean
Take along flip-flops for the shower and deodorant for after, and a hairbrush, hair elastics and sweat-absorbing headband. Add travel-sized bottles of shampoo and conditioner. Even if your gym provides these things in its showers, you could be allergic to them and it’s more comfortable to have your own products. Include razors and any important cosmetics or toiletries in small packages. Pack aspirin, so a random headache doesn’t spoil your workout.