We keep our meds locked in a cabinet that is mounted on the wall. It is located where we administer the meds- in our case it's a little kiosk near the dining hall. That way we don't have to transport the meds anywhere, the kids come to us. If you don't have many meds, and want it to be portable, you could get a locking file cabinet that has wheels and a handle. Even having each kid's ziploc in an alphabetized folder works. I have even heard of school nurses that use the file folder method.
For activity area first aid kits, each area has a lightweight drawstring backpack that we fill and replenish as needed, and it just stays in the area, verses counselor having to carry things. Fanny packs work well, too. That is what our lifeguards use. For the activity areas I just provide the basics. We found when we made first aid kits too fancy, the stuff just ended up getting wasted. So we keep it pretty simple. Plenty of gloves, BZK wipes, sting wipes, and bandaids. A baggie of gauze in case they need to hold pressure on something.
Our nurses carry a backpack with more extensive emergency and first aid supplies.