I would say if you want something noone will expect. You could make the party a surprise party, but instead of being a surprise party for yourself, make it a surprise for all your guests!
You could invite them all individually to hang out, but make sure they don't know you've invited the other people. I guess you could do it in small groups too if it gets too complicated. Say something like "hey I only want to have a small party with just you guys", but say it to like 8 or 9 different groups. Then when they all show up, you could surprise them with a big bash.
As for the venue, you'll need to pick something they won't expect and somewhere where they won't have a clue what's going on until they enter the room.
for extra mystery, get them to all wear masks when they come in. And hire a dj and have a smoke machine too!
