Anyone, at any time, can call out “rotate” and everyone in the room has to get up from where they are sitting, and move to the chair to their left. I have no evidence to prove it, but I’m convinced moving around physically helps people to move around mentally.
Tell the group there is no limit on costs, or time, or other resources. They can think as big or expensive as their minds allow. Think of other constraints that the group is assuming. Business? Political? Customer? Take the biggest ones you can find and get rid of them. Removing a roadblock might free new directions of ideas that wouldn’t have been considered otherwise.
To get some creative momentum, my first activity is typically “list things you can do with a brick”. It’s a well-known creativity exercise where you put one idea per sticky note and see how many you can come up with (e.g. throw it, prop open a door, make a wall, etc) But the key here is to make everyone do their own sticky notes. In silence. For 5 minutes.
To maintain the group’s focus, create a parking lot: Unrelated issues can easily divert discussions, and while it may lead to exciting and viable ideas, you should review these at a later date.
Set Limitations - Without the pressure of time or money, your team can be left languishing during its brainstorming sessions. Use constraints such as deadlines to drive both creativity and productivity. In the best brainstorms, people feel pressure to show off what they know and how skilled they are at building on others' ideas. But people are also competitive in a paradoxical way. They "compete" to get everyone else to contribute, to make everyone feel like part of the group, and to treat everyone as collaborators toward a common goal.