While there are any number of national organizations to choose from, there are probably plenty in your host city that would benefit greatly from your donation.
Auction off those beautiful centerpieces from your evening reception or hold a 50/50 raffle at your closing session and donate the proceeds. Put on a fun-run or 5K for your participants and donate the registration fees. Then encourage locals to volunteer along the race route.
The opportunities to give back are endless and by choosing an organization that best fits your attendees you’ll find it’s a great way to get them to come together.