Speaking of intermingling, games like bunco don't require much planning, are easy to learn and play, and are very fun. Bunco is so easy to play, and can be played with big groups too! How to play bunco
You can include video games too - Rockband, Wii, Nintendo...just make sure you have enough for everyone to do, so it isn't just a couple of players with everyone else watching. That would be b-o-r-i-n-g!
How about finishing the day by breaking into teams for a scavenger hunt? Scavenger hunts and treasure hunts can be totally customized, and are always a good time. The catch here is that everyone breaks up into teams and they won't be around each other except at the beginning and end of the party.