There's a classic saying by Dale Carnegie that "People support a world they help create."
If you want to create a sense of ownership and accountability, and create a culture of openness and transparency, invite the direct reports from Day 1 !
If they are involved in the process from the beginning, they will feel a sense of ownership and help drive the process. That's how you create the next generation of leaders -- invite them into the inner sanctum and the inner circle, make them feel like the leaders or future leaders that they are, and then challenge and empower them to step up to leadership.
The second thing, as some of my colleagues mentioned, is to bring in a seasoned, experienced external facilitator to run the session. (And I'm not just saying that because I am a seasoned, experienced, external facilitator :) Seriously, you want to allow everyone including the CEO and senior leaders to be participants in the process and not be concerned with running the show. What that also does is level the playing field and encourage others to contribute their thoughts without concern for hierarchy, status, or title.
Lots more can be said, but I'll leave it here...except for one other thing: Before leaving, make sure you capture the Action Items and next steps by determining and documenting Who is committed to doing What, by When. I've seen too much good work done in off-sites gone to waste when everyone returns to work and dives back into business as usual and focusing on nothing but playing catch-up on everything they missed while they were away. What you want is to keep the momentum created at the off-site going strong as you return to work energized and excited, ready to hit the ground running, and with a plan for how you are going to turn all those big ideas into actions, and actions into results.