Letters can be arranged into words, so that's a start.

articles that might interest her (you're already doing this)

artwork (You could buy an adult coloring book and send her pictures you colored yourself. I always wanted an adult coloring book :-))

Does she like, or care, about beauty products in basic training? If so, you could send her those flat samples of perfumed lotion, face serum, hair stuff, etc.

poetry

I've always loved the Dover stained glass coloring books.

I like to use a lot of stickers and cutouts when writing letters. Even if they have nothing to do with the topic, they're still fun.

Fun quizzes, like you find in magazines, but make them personalized for her.

Things like:
What is your favorite flavor of ice cream?
Are you a night owl or an early bird?
Who is your hero?

Include a stamped envelope so she can return it to you.

You could make some of them multiple choice so she could just circle the answers. Then you could send it back with YOUR answers circled in a different color ink and she could keep it.

comic strips from the newspaper

Are there any websites that would have funny things I could print out for her and include with my letters?

There are lots of comic strips. Maybe some younger, hipper folks could suggest others. I'm assuming your friend is young. She may not dig Cathy and Roz Chast. There is Metafilter's Greg Nog. I love him! haha

I like to use a lot of stickers and cutouts when writing letters.

Yeah, stickers! Great idea. And pretty or funny notecards if she's into that kind of stuff.

pretend ribbons and medals? "Most likely to make E-3"
Snowball Campaign Ribbon with star?
Nut Bar with Clusters?

Military decorations have a curious logic to them, and are always surpassingly serious and grave. You could certainly do some of your own (on paper, or with embroidery floss, or however fancy you want to get) with her, specifically, in mind.

Here are some examples.

Don't send gum, money, anything that might be considered porn, etc. Mostly, send mail often. I kept up a steady stream of mail when my son was in Basic. I sent newspaper clippings, cartoons, and lots of boring stories about my day, the weather, the neighbors, etc. He loved it and really looked forward to mail time. Don't include stressful stories. I did have some good mail about politics, but not wildly controversial.

Thanks for all the fun ideas! I will be referring to this thread often.

Seconding Theora55, they are pretty tough on you in bootcamp in terms of whats OK and whats not. Getting mail is normal, getting a package means having your CO watch you open it and decide if you can keep it and whether you need to do more pushups for no particular reason other than coming to his/her attention. My mom sent me a crossword puzzle which I put in my pocket (we got our mail outside while on a break), my CO found it later during a random inspection and that was 50 pushups. Basically, anything that doesn't fit in a regular envelope will bring your friend unwanted scrutiny. When they get out of basic, they won't have these kind of restrictions.