How do you prequalify them? Can you just cold email/call? Do you get in touch with HR or someone else? What do companies look for in a corporate retreat?
I would say Email Outreach is a good input strategy for your funnel and HR or Business directors are your best bets.
a great sales funnel for this would be
Being email outreach
A great visually stunning Landing page with a Facebook pixel
The 3rd level:
Re-marketing Ads to advertises to the interested people who click.
Hope this helps