Extra caffeine crystals in the morning beverage, seeing as it's tasteless.
Or Sodium monofluoroacetate in just about anything ingested or applied to mucous membranes - I've always been convinced that this was the basis for 'iocaine powder' in the Princess Bride movie.
Manchineel pieces in the fruity dessert to make his throat close up.
Or enterotoxic E.coli in the burger. To shrivel his kidneys and make the world fall out of his bottom.
Or just defenestration.
...What? Wikipedia is great!