You could try working with themes. You could make a list of possible themes that would be of interest to your group, and let them choose the ones they'd like to work on. That would give you a focus for working on.
When you have the themes decided, you could look at what types of language and activities might fall under that theme. For example, if the theme is "Food", you could look at vocabulary for food and cooking. Grammar could be count/non count nouns, partitives... Speaking could include dialogues for ordering in restaurants, shopping, giving instructions. Students could write their favorite recipe, or write a restaurant review.
You could end the unit with a potluck party with the class.
For any theme, you would choose related activities and build on those. It can work well with multi-level groups.
I hope this makes sense!