I love doing sorting, and love doing it for observations! It's so fun for kids. Here's my favorite sorting activity:
Have the kids sit in a big circle and put two hula hoops in the middle. Give everyone an object to be sorted, and tell them how they will be sorted. Start simple and start with two categories that all the objects fit in. The best things for this, if you have them, are attribute shapes.
Next, make the categories a little more complicated. For example, one hoop is triangles and one hoop is red shapes. Make sure there are a few red triangles, and you give them to the kids that think outside the box.

Choose those kids last, and let them figure out how they can sort them when they belong in both categories. 99% of the time they will figure out on their own how a venn diagram works.
Last, do it again but give a few kids objects that don't fit in any category. See if anyone realizes that you can put them outside of the hoops.
Totally interactive and shows lots of levels of thinking. I always love it and so does admin!
ETA another fun activity is to sort the kids. First with known categories, then I pull some kids up and they have to figure out how I sorted them.