Give the small things a home
A bag becomes annoying when the entry card is underneath the shoes and the locker key has slipped into a towel. Reserve one reachable pocket for the things you need before exercising: your access card, lock if required and any small personal essentials. Keep that arrangement consistent. The point is to avoid emptying the whole bag at reception.
Pack for the facilities you actually use. Someone changing at home needs a different load from someone showering before work. Check whether the gym supplies towels and whether its lockers take a padlock, coin or code. You do not need to purchase a collection of accessories to attend an ordinary induction.
Put shoes in a separate washable compartment or shoe bag, especially after a wet journey. Keep clean clothing away from damp items. A reusable bag for a used towel is useful on the trip home, but take everything out to dry when you arrive. A sealed compartment is transport, not storage for the rest of the week.
Think through leaving the building. If you will shower, can you reach your clean clothes without placing them on a wet bench? If you are going straight to work, are your keys and travel pass still accessible? Those questions often reveal a missing item more effectively than a long online shopping checklist.
A bottle that closes securely is easier to live with than one that leaks onto clothes. Put toiletries in a pouch and check caps before packing. Avoid carrying valuables that you do not need; follow the facility's storage guidance for the things you do bring.
After two or three visits, remove what you have carried without using. Keep a spare only where forgetting the item would genuinely disrupt your visit. A second pair of socks may earn its space; several unused resistance accessories probably will not. Before leaving home, a quick check for entry, clothing and water is easier than maintaining an elaborate packing ritual.