We've found agencies have the most success conducting Plan sessions at their own offices vs. going to the client's office. At the client's office, the client could easily get pulled out of the conference room to take a call, or weigh in on something “really quick” and you lose momentum and focus. Also, clients tend to love getting out of their space and going to the "cool agency space." A conference room is ideal - just any place that can comfortably accommodate the number of guests, plus your two team members.
You’ll want to find a room with a large wall space and seating area, where the entire wall is visible. You’ll want to post the category statements on the wall and be sure the wall is accessible so team members can physically reach the posted statement and tape their ideas for tactics to each sheet.