I empathise as I can do small for personal trips but struggle massively for Cub Scout trips....(and as the "Mammy" Leader, I also end up carrying the spare wet jacket and sleeping bag and blankets for Cubs who forget or have theirs thrown in the lake....
). Luckily, I normally drive my estate car to camp....
OK, to be serious.
Wear Uniform travelling. Wear boots if you are bringing them, or decent trainers.
Actually I just googled uniform - it's really changed!! So wear a pair of either sports leggings or hiking type trousers under the tshirt and hoodie over your shoulders.
Pack:
3 knickers (2 to wear, 1 spare) and at least 4 pairs socks (you ALWAYS need spare socks!).
2 or 3 tshirts - rolled.
Spare bra.
1 spare trousers.
Have your pac-a-mac trousers in there in case of rain.
1 extra warm layer (a long sleeved top to wear under tshirt in evenings, or an actual thermal layer).
And a hat if you will be outdoors in the evening (are you camping? Then DEFINITELY) - may also want a sun hat if sun expected, but could wear this while travelling.
PJs.
Wash kit - toothbrush and paste, shower gel, shampoo (and conditioner if you really need it), moisturizer, deodorant, sun cream. If possible at least decant into airline sized bottles (I find 30ml size is fine for a weekend, I only use 100ml for a week or longer trips). Preferable is any sachets you have scrounged from magazines or similar - shampoo and conditioner, shower gel, body lotion, sun cream etc.
Hairbrush.
Towel - a camping style one or hamam version as they take up a LOT less space.
You might want a book for some downtime/while you wait for the darlings to settle to sleep before you can retire - always late on night 1, FAR earlier on night 2!).
Small Day pack includes Torch, small first aid kit, whistle (VITAL!), matches for campfire/gas stoves, rain jacket can go in there if you're not wearing it, snacks, water bottle, phone charger etc., as well as any paperwork you need (including a list, on paper, of the contact details of each girl going on the trip at least even if you don't have a contact list printed off for the whole Group generally). (Compass, small swiss army style knife, map, pack of cards, pencils and small notebook or blank paper are useful optional bits for the daybag.)