One card sort I tried for the first time was Similar and Congruent from The Dog Days of Math Blog (thank you!).
I gave each student a mini deck of these cards in which they were given about 5-10 min to decide how to group them into two groups. They needed to come up with properties or explanations to why they decided to group them that way. It was a great quick thinking and discussion activity before formally defining the terms.
A fun warm up activity that allowed students to have some ownership and connection with the definitions.