You're right, this is the multi-million dollar question! Dealing with differentiation is one of the hardest things about teaching in my opinion, and you're not going to be able to get a magic answer from a forum.
However, you might get some good ideas of where to start.
I've dealt with mixed ability classes and the best advice I can think of is to work thematically with them on a project that enables them all to work at their own level. So for example pick a broad topic like 'animals' or 'my family' and have them make posters/books/presentations/youtube movies/blogs about their topic. You can have them all working on their projects and rotate around the class working with a group at a time. So you might say to a more advanced group that you are looking for them to use the present simple to give factual statements (e.g 'Spiders are not insects, they are arachnids') or you want them to use the past simple to describe family histories (e.g 'My mother was born in...she grew up in....), or that you want them to use persuasive language (e.g 'Although some people believe that we should resurrect dinosaurs, I believe that would be very dangerous because...'). For less proficient students, you could focus on vocabulary and very simple sentence structures. If they are just beginning to write you could just have them label pictures. If students are working on a topic they chose themselves, they are much less likely to get bored.