S.B.
Buy a nice hooded white sweatshirt and let everyone sign it with a permanent marker at the celebration. That way, the girl leaving can "wear" her friends with her for a long time.
I did this for my daughter and she's 24 now. She still has the sweatshirt.
You can still do the t-shirts so the other girls have her signature too, but I was just thinking of logistics.
How about starting " friendship journals"....notebooks even from the dollar store.
Write each girl's name on one.
Pass them around and let the other girls write their names, addresses, birthdays, phone numbers, messages, put stickers, draw pictures, etc. Then, each girl gets hers to take home at the end of the meeting.
That way, they all have a little momento and perhaps can stay in touch with each other outside of the meetings. It's kind of the same thing as a memory book idea, but the whole troop can share as a friendship project.
Also, you can buy plain pre-post paid post cards. Let each girl write her name and address on it, decorate it and give it to the girl leaving so she can write a note to her friends once she gets settled in her new home and town.
Pen pal projects are great for kids. Keeping in touch through the mail is becoming a lost art. I always encourage it.
I hope you get some great ideas!
I was a Brownie leader and absolutely loved it. It's so much fun!