I grew up in the middle of nowhere on a farm. Here's some things I used to do as a teen/stuff I'd probably do as a teen if that technology was invented back then:
-The "Be a Tourist In Your Own Town Challenge". If you're able to get into your local town with a camera (either mobile phone camera or digital camera) to take photos of random things that a stranger would find interesting. For example, where I live we have a giant office in town that looks like a milk carton. I found out the other day that we have a metal sculpture in the dodgy side of town of a clothes lines with clothes on (I'm so proud of my town sometimes...)
If you can't get into town, go on a walk around your area and take snaps of stuff you find interesting.
-Make your own skits/videos. Whether it be something funny or not. Write a small script, dress up as all the characters and act it out. There is usually free editing software on your compute somewhere that'll do the basic job. You don't have to post the videos anywhere. (I once made a comedy skit of the 'Gay Agenda', dressing up as a spy and the opening scene was me petting a cat and calling it Mr Bigglesworth with plans to take over the world. It was hell fun! The crazier the more fun it is)
-Learn a new language. Heaps of online tutorials and videos. Fyi, semi fluent in Japanese person here 😉
-Try a new hobby like yoga, tai chi, running/ something that doesn't require equipment. Again, an abundance of online tutorials.
-Star gazing. Get to know the star constellations via the internet and try and find them in the night sky. Certain times of the year some planets come into view.
-Write a bucket list of things you want to do in life. Then plan out how you would do them
-If you have the cash, scrapbooking is an awesome way to pass the time.
-And if all else fails, debate people online. If you're passionate about a topic, there is bound to be someone on Facebook who disagrees with you. Find them, and debate them. Coming from someone who does this all the time (Sometimes I wonder what I'm doing with my life)
Hope that helps. Let me know if you try of them!
Edit because I thought of more things.
-Start a "Box of Memories". Maybe it's because I'm a hoarder, but stuff like concert ticket stubs, notes from friends. Make a box for it all. I started one when I was 17 and got things from in high school in there.
-Start a journal
-Make a collage board- photos you like, pictures of things you like from magazines, etc. Get creative.
I'm sure I do have more. My memory on my teenage years is slightly fuzzy 😛