Before a student gets to the point of overload a well timed break from the demands of work can do wonders. These breaks can be structured into your teaching for the whole class or can be offered to individual students as required. Be sure to structure these in before the students behaviour is getting out of hand, you do not want this to become a reward for inappropriate behaviour, rather it needs to be structured as something that supports the student so they are able to control their behaviour before it becomes inappropriate.
Some suggestions:
Individual student
Movement break
Drawing
Colouring
Reading a book
Go on a message
Play dough
Puzzle
Kinetic sand
A quick activity of interest
Whole Class
Minute to Win It
Heads Down, Thumbs Up
Silent Ball
Movement Break - Just Dance, Go Noodle, Atom, aerobics, balloon volleyball, yoga
Ongoing project - puzzle, origami
Go outside for a walk
Calming music/videos
Meditation
Colouring, drawing or reading