I guess it depends on the colour scheme. I went to a wedding once that was red and gold themed, they had a single tier cake too and a simple decoration on top. It was just a little arrangement of red and gold roses made out of icing. It was small and understated but it worked really well, as it fitted in with the theme.
If you just don't like the 'usual' bride and groom setup, my mum had a great topper specially made of her dragging her groom-to-be by the collar back to the altar. It was a brilliant talking point and we still have it on display in our house today. They even got her husband's bald spot!