Number talks are terrific activities for developing mental math skills while encouraging students to talk about the ideas they used to solve a problem.
Background
Resources for Math Talks
http://www.sfusdmath.org/math-talks-resources.html
Introduction to Number Talks (Youcubed video)
https://www.youcubed.org/resources/stanford-onlines-learn-math-teachers-parents-number-talks/
"Number Talks" to Grow Mathematical Minds
https://www.scholastic.com/teachers/blog-posts/alycia-zimmerman/number-talks-grow-mathematical-minds/
Number Talks Toolkit
http://www.mathperspectives.com/num_talks.html
Number Talks Video Examples
http://www.insidemathematics.org/classroom-videos/number-talks
Other Routines
Routines for Reasoning
http://www.fosteringmathpractices.com/routinesforreasoning/
Number String Routines
https://numberstrings.com/
Clothesline Math
https://clotheslinemath.com/
Which One Doesn't Belong?
http://wodb.ca/
Estimation 180
http://www.estimation180.com/
My Favorite No
https://www.learning-space.org/Page/94
Notice and Wonder
https://www.learning-space.org/Page/96
Would You Rather ...? Choose a Path. Justify It.
http://www.wouldyourathermath.com/