I learned about this activity at a training last week. Basically students look at three objects, numbers, shapes, etc. (it can be very flexible) and decide which one does not belong and why. This activity is amazing for several reasons.
1) It is accessible to students at any level and all students can feel successful.
2) It naturally encourages students to think deeply and creatively.
3) It can be done repeatedly over time and the students' thinking will get better and better.
OK so here's how it works:
Start with 3 pictures of different animals or insects (something that is easily accessible to ALL students no matter what their math knowledge). Give students quiet think time to think about which one doesn't belong. After 1 minute, allow them time to discuss with a partner which one doesn't belong and why.
Then after 2 minutes have students share their ideas with the class. It's awesome to see all the different ways that students will interpret the pictures.
Next repeat the same process with math numbers. 1) Quiet think time 2) Partner sharing 3) Whole group sharing.
Some concepts that your students may bring up: even, odd, greater than, less than, multiples, value, etc.
This can be practiced over and over again using digits, ten frames, coins, shapes, and much much more!!!