Total cost and a line-item breakdown of what's included.
Amount of deposit and when it was paid.
Balance outstanding and when it's due (a payment schedule).
Exact date and time of your party.
Exact location of your affair (e.g., "In Main Gallery," "In Presidential Ballroom").
Detailed list of everything the place will provide (tables, chairs, linens, amplifiers, whatever).
The name of the site representative who will be on hand the day of your event, and the name of an acceptable substitute.
Proof of liability insurance and liquor license.
Cancellation/refund policy.
Anything else you agree to orally that you want to set in stone.