A scrapbook sounds excellent. You can add all kinds of bits and pieces. You can write entries, then put something near it to remind you like: movie ticket stubs, pressed flowers, pictures, fabric swatches (that can actually be cool, like if a certain event happened based around something someone wore), ribbon, stickers, etc. Be creative.
I know what it's like to have an excellent friend move away. You should do something together before she goes. A whole girls' day. Maybe go see a movie, shop, go out to eat, have a sleepover, whatever you two enjoy doing together. A big blast before she leaves. Just remember to keep in touch afterwards.
Ooh, another good present could be something to keep in touch with, like a stationary set and a ton of stamps. Maybe even a decorative pen. Let us know what you finally decide on. I love to hear about thoughtful presents.