Take a regular SIOP strategy, Four Corners, and instead of using paper, use iPads. Possible apps are Explain Everything, Educreations, Edmodo and other collaborative apps that can document student work.
In their corners, students pair and explain why they chose that subject and what they know about the topic. Now you can decide if it's the beginning activity, they can just write what they already know. To extend the activity they could research and gather information via the Internet to post.
This activity is also a method for creating voluntary groups. After the Four Corners technique is over, you may want the students to document their discussion and the pictures they gathered,or information for another group task.
VARIATION TO BRAINSTORM: Post different concepts around the room, have students travel to add information or list everything they know about it.