Just subscribed to this blog:
Unusual Businesses Ideas That Work
The blog lists existing businesses only, but here's a couple of rather wacky business ideas that spring to mind:
1) Sell the contents of the discarded vacuum cleaner bags from the vacuum cleaners used to clean Buckingham Palace. (I heard that house dust consists of ninety per cent dead skin -- so if you're buying the contents of the cleaner bags you're buying a piece of the Queen!)
2) (For the Tower Bridge souvenir shop): Make transparent plastic models of Tower Bridge and fill the models with genuine Thames water.
I'm sure I've got some more ideas for unusual businesses in 'ye olde files'. Must dig them out some time...