Any event that a business puts on is ultimately a marketing strategy - whether internal (employee-oriented generally for morale or gratitude purposes) or external (client-/prospect-oriented generally for gratitude or business growth). Examples include:
The hardest part about a successful marketing event is making the marketing aspect of it so well-incorporated into the event that your attendees don't really notice it.
There is some finesse involved. You don't want to lose the marketing message while you work to make the event fun and talked about in a positive way for business.
Make sure any free food and booze is an element of the event; not the main attraction so your marketing message isn't lost!
Keep it fun, but always respect the reason for the event and your goals.
And remember, a marketing event is a business event - drunkeness should never be the goal.