A great way to engage students is to create projects within contexts and formats that they are already using in their personal lives. One of my favorite ways of doing this is through blogs. Teenage students already spend a lot of time online, especially on social media sites, so why not encourage them to continue their online interactions, just in English instead?
I welcome my students to write about topics they already love, which means their more likely to put in solid effort to the project. I tell them at the beginning of the year: "Your project is to write about whatever you want to write about." If you want to narrow the focus or if you think your students need more structure, you can choose a topic and/or style for them. Or perhaps each post can be a response to what they've been learning in school the past month. At the bottom of this post you'll find a download link for topic and style ideas.
Set aside rules for your students. I asked mine to submit two posts per month and set a word limit for them (250 words minimum for my 6th-graders, 350 for my 8th-graders). They also needed to comment on at least 3 of their classmates blogs each month. Of course, they shouldn't write anything you deem inappropriate (and you'll need to discuss that with them at the beginning). You can check out my sample rules below, but you should make adjustments to tailor to your class's abilities and interests.
Every month, I selected a blogger-of-the-month and awarded that student a prize.
I found blogging to be a great activity for multiple reasons: 1) Students practice and improve various writing skills, including forming strong sentences, spelling correctly, and formatting paragraphs properly. 2) Students want to improve their grammar so their blog doesn't look bad in front of their other students. 3) Students who love to talk now have the freedom to talk (write) as much as they want. 4) Students who tend to stay quiet during class now can express themselves in different ways, free from interruptions and with the time to think about what they want to say. 5) Students' vocabulary increase as they try to find the right translations for whatever words are on their minds; if they're excited about the topic, they're more likely to remember those words later. 6) Through their comments, students practice exchanges such as disagreement, approval, brainstorming, and more. 7) Depending on the style your students do, they may have to learn how to research their topic.
If you want to help your students improve not just their writing but their general English skills while doing things they already do, I can't recommend blogs enough. Download the documents linked below for more ideas.
By the way, I recommend finding a site that allows closed communal blogging. If you prefer, you can prevent the general public from accessing the blogs. You'll also need to check the blogs regularly not only to make sure their meeting the requirements but also to ensure the students are civil with each other. I highly recommend blogging for teenagers, but of course adults may enjoy doing this as well.