What makes an online course a valuable learning experience for a student?
If your course delivery platform has things like forums or collaborative writing spaces, you should make use of them to get students interacting with each other.  I don't know what discipline you're in, but you can ask students to comment on the readings online, write responses to each other, that kind of thing.

I was teaching a regular course but was ill for a week, and not wanting to miss that whole week of class, I scheduled a live chat during the time the class would ordinarily meet.  I think we had the best discussion of the semester that day -- students who wouldn't ordinarily speak up in class "spoke" during the online chat.

Also, set boundaries for yourself; otherwise you'll find yourself answering emails at midnight in your bunny slippers.  Tell students if they email you after a certain time, they will get a response the next day.  And think about how you feel about responding on the weekend.

What is your discipline?  We might have more tips, depending on what it is.