That are worth something and intresting
None.
You’re asking the wrong question.You should be asking what are good activities for you? And nobody can answer that without knowing your interests.
Running your own legal and profitable online business? Mastering computer programming in preparation for the Computer Science Olympiad? (that’s not entirely online strictly speaking, but close enough)
Why “online”?
There are a lot of things that you can do online. Sometimes organizations have national youth advisory boards that only meet online, that kind of thing. It might not be advertised as "online’ but when you look at the details it will be explicit. Do you go to online school? Online schools have great online ECs. If you study a foreign language, developing a Skype or pen pal relationship to practice is good. Also, you can get involved in volunteering online. Some organizations might have opportunities for you to provide advice, support on a peer basis once you’ve been trained. In your own community, you could create a resource page on a topic of interest to you, either on FB or as a website/blog.
If you like history:
http://mentalfloss.com/article/561842/library-congress-needs-help-transcribing-lincolns-letters-and-other-historic
Like any business related programs online that are accredited
the one that you enjoy doing