Note: These are not strict rules. They are interchangeable, but usually will hold these meanings.
As implies they are wearing clown attire.
They were all dressed as clowns with big red noses and over-sized pants.
Like implies they may be wearing something that looks goofy or resembles a clown, without actually wearing clown attire.
They were dressed like clowns in their unorthodox clothes.
With your bomb sentence, as and like convey a very similar meaning. When using as, you are saying the bomb literally looks like a package. When you use like, the bomb has characteristics that it shares with a package, but can be differentiated from a package. For example, the bomb may have a cardboard exterior like a package, but has buttons on top for timing the bomb as well.