Take heavy doses of B complex. Can't guarantee nightmare, but you'll increase dream recall.
Cheese have been proven to enduce weird dreams if eaten before bed
Don't know why you would actually want to have one, but I know when I have to pee really bad I have some terrible nightmares.
You want nightmares? I get them almost every night and I want to have normal dreams again:/
eat chocolate before bed
then you must be fearless
Those are usually all I have.