Heather McKay & Abigail Tom (1999, CUP, Teaching Adult Second Language Learners, pp. 21-22) suggest teachers differentiate among four types of mixed-ability activity. Unless the text is in quotation marks, it is my own interpretation.
same input, same task
What is different in this situation is the level of your students’ language proficiency. What makes it possible for the students to do the task is their collaborative effort. You have to divide your class of students into pairs or groups so that weaker students get to work with stronger ones. The tasks that are best suited in this case are those that require problem-solving skills, e.g. games, puzzles, mazes, quests, trivia quizzes and the like. In other words, the focus is not on English but on the task, which should require the students to draw on their knowledge of the world and life or work experience as opposed to their knowledge of grammar rules or lexis. You should design the activity so that it would not look, feel or sound like a language practice activity.
same input, modified task
A good example of such an activity would be a multilevel dictation. The more proficient students would have to write everything, the less able ones would have to fill in the gaps, and those you consider a pain in the neck could be asked to tick the options they hear. Once you have finished dictating, everyone should have the same text.
Another example is multilevel listening. Stronger students may be instructed to listen without reading the script while the audio is being played, and weaker ones could be permitted to consult the script as they listen.
By and large, weaker students are provided with more scaffolding.
different input, same task
This type of mixed-ability activity requires weaker students to use the input you provide “as is” and stronger students to do something with the initial input in order to do the main task. For instance, you can choose to give the more proficient students in your class cues and the less proficient ones ready-made questions when you do a mingling activity such as “Find Someone Who”.
same task, different performance level
This last type is very much like project work. What makes it special is that the teacher doesn’t give out any materials, but just sets the task. The students work alone or in small groups, and the language they produce will vary according to their level. I imagine all sorts of “create a poster” type of tasks will fit in this category.