Adult parties can be as fun or as formal as you want. Depending on the crowd you might want a super fun childish event with all the works, magicians, face painters, games, etc. If your crowd is a little more reserved, an event with less pomp and circumstance may be a better option. Find the party that is ideal for your group and either go all out with decor and activities, or keep it to a minimum with just a slight hint of a theme. What matters is that you and your guests have a good time!