A surprise party.....my husband did that for my 30th and it was brilliant, He went through my email address book etc and got hold of old uni mates, people I hadn't seen for years, as well as inviting family, friends, neighours etc.
He hired the function room in our local and said we were going out for a meal, everyone was waiting in the bar when I walked in (I had no idea). He had organised a bit of food and some music, people bought me drinks all night and it was fantastic!