Well, I think the key is to make the writing unpredictable. To achieve this, you can write about something small, like giving the ants in your yard a piece of apple, or write about multiple things and mix them together so nobody knows what's going on. For my song Hazard, I wrote about 3 different girls and just mixed and matched feelings. This made people go "Is this song about a person or a monster?"
Also, stream of consciousness singing is fun stuff. Listen to Underworld sometime. Their lyrics are all over the place and seemingly about nothing, but subconsciously, they make sense.