I have a "Kids Sports Bag" which has everything from the equipment needed for the current sport to bandaids and sunglasses. We used to add things as the season went on but now we just keep it stocked. My son helped me write out a list to be sure it was stocked. Right now it has:
5. protective cup
11. emergency cash (for French Fry Emergencies)
14. wet wipes
16. water bottle
17. (later, sunscreen – I feel optimistic that we'll need it sometime soon!)
Now I can grab my keys and my phone and we're out the door. When I wash the uniform it goes right back into the bag.
I had to regularly run around in a panic before swim lessons (where are the goggles! the swimsuit's still wet! we're out of sunscreen! etc.) before I stumbled upon this hack myself last summer. Such a sanity saver. In fact, it's worth generalizing this idea for all sorts of activities: picnic bag, hike bag, beach bag, "waiting around" bag (because you're bound to hit one of those killer dead spaces in your week, and certain little kids just can't handle them without distraction), etc.
Any other ideas for handy pre-packed bags?