In case it is only for employees,you have to follow a different concept and if families are involved then a different one. If only employees are invited you can have a play related to the organisation(fun), can have some open discussions, and some management games too.
If families are involved you can arrange for some celebrity (within limits), arange few games for children, dance floor for couples and the organisation concept in a simple way in form of games so that families can take part (i have done this) ...it works!!!
Naming the event is obviously the most important thing..which attracts people...
Finally a thank you gift..if your budget allows!!!!
Hope it works...