I love Targetboards as a mental maths warmup. In fact, one could have a Targetboard to start off any Maths class. Bright Sparks has created a lovely Targetboard with questions for the teacher, which is well worth downloading.
She says: Use these target boards for mental maths activities. There are 3 target boards. Each one is easily adaptable to suit the needs of your class. Each target board comes with 20 questions. From easy to more challenging.