Some other ideas for starters:
Mental Tests, not just arithmetic but on any topic. Students must listen to your questions carefully and write down the answers. CIMT have Mental Tests on every topic in their teacher resources, see the Year 7 page for example (scroll down, the Teacher Support material follows the content for each book), this is an example of a mental test.
Give them a diagram – they write the question!
Brain Food is mentioned on the Number page – note the other problems available, a Logi-Number puzzle could be written up quicklyand would make a good starter.
Arithmagons can make great starters for any age group. A sign of a good mathematician is the ability to work from the answer to the question. For an introduction see Nrich on arithmagons for something a little more advanced see Advanced Arithmagons from Jonny Griffiths.
Visualisation – for example ask students to imagine a coordinate grid and give them some points to plot on their imaginary grid. What shape have they just plotted or what line do their points lie on?
There is an OU OpenLearn study unit on using visualisation in maths teaching.
Perhaps a little Geometry, for example – what can you say about a rectangle inscribed in a circle of given circumference?
You could then check the result on WolframAlpha – see this recent blog post.
On the subject of WolframAlpha – some fun facts might make an ideal starter.