I work with middle and high school students who want to learn how to write skillfully--to handle more than traditional school assignments. Among the activities we explore is blogging on Tumblr. Access is free, the kids can choose anonymity if they and their parents prefer, and they learn to talk to a real audience about what they know and care about. One 16-year-old designed her blog--Hello from the Science Side--for 14- to 17-year-old girls, to encourage them to stick with STEM subjects despite social pressure to take less difficult learning paths. To get ready, we read Natalie Angier's The Canon together to learn techniques for telling stories about subjects ranging from statistics (For Whom the Bell Curves) to Physics (I've Got Plenty of Nothing). The results have been beautiful, not just because of the writing, but because the kids have to learn HTML, figure out the metadata of hashtags, discover Creative Commons graphics, and much more.
IDEO.org's Frame Your Own Design Challenge materials are a great resource for thinking through how to pick the right focus for the blog.
In brief, blogs are easy to set up, free, deeply creative, and fantastic for college applications. Happy to discuss further!
Just realized that you want to know about the thorns, too: Blog posts take time to write and edit. The student, parents, and you (!) need to be committed. One post per month may be all that's possible.