Type of event - Is this event suited to drinks in a bar? A buffet? Or perhaps a sit down dinner? Will this event take place during the day time? You might like to consider some kind of entertainment and perhaps a photographer to remember the event?
Costs - Remember that you will need to take all costs into consideration - catering, venue hire, accommodation and any entertainment.
Payment- If you are planning to charge individuals for their attendance make sure you plan how you would like to collect the money.
Tickets - Will attendees require a ticket for the event?