A classroom activity that I think students would enjoy with encoding and decoding is making a cipher.  This entails that students can either make up their own coding system or use the binary system and come up with coded messages for others in the class.  These coded messages can be a single message or an action that they need to do after they have worked it out.

If I was teaching this online, I could still do this activity I would just need to figure out if I would do it in groups or individually.