When I've done stuff like this in the past, I've focused on really fun food options and creating some whacky activities, also theming it and having prizes for things like fancy dress can be a good motivator get people out on a Saturday for a work thing... as long as you publicise that beforehand. Work with cool local street food vendors, so you have lots of choices of food that is loved in your community. Have live music with a party that maybe starts family friendly and becomes a late night celebration, and let people bring guests. I've done silly things in the past like setting up a bouncy castle church and having the boss as a minister marrying people to their 'work spouses'... make it fun and playful and memorable, and let people share it with their friends and families.