Whiteboard is the way to go. It’s spacious enough for several people to work on, it’s flexible enough to make changes throughout the exercise and it’s a good place to capture it on to paper from after the session is over.
One of the best mindmaps I’ve seen had a lot of images attached to the concepts it was mapping – if you have someone who’s good at drawing in your group, get them involved in making the final version. Mindmaps are very visually oriented tools and these little sketches made it so easy to follow and grasp – I’d highly recommend it!