Let's start with some classics:
>One potion will just switch your sex.
>One potion will completely nullify your Inertia for 20 minutes.
>One potion will double your weight every 6 hours.
>One potion will make the drinker no longer dependant on water to survive, but he will instead need alcohol. (It will still get you drunk)
>One potion will make you grow an extra eye in the forefront.
Other kinds:
>The potion may seem without effect at first, but after a while, they will feel a really strong urge to defecate, the fecal matter will be normal, but after 1 hour it will start to grow and become anthropomorphic, if not stopped, after half a hour it will become one exact copy of the drinker, with all his memories, stats and such.
>The potion will make the drinker look upon the sky, always smiling, but not in a way to would make you unable/hard to talk, every phrase the drinker would say would have an over the top happy and laughing tone. The drinker hands will lay on his low torso, in a satisfied-like manner, and they wouldn't be able to be moved for 1d6 days.
>This potion will make everything the drinker says useless and unimportant on other people ears, and every single feat he will make for the next 1d6 days, no matter how difficult or heroic it would be, it will seem like child's play in the eyes of everyone witnessing it, even if it, for example, involves slaying a god.
>This potion will make the drinker aware of the location of a treasure, and it will make you do everything to get to that treasure, the only problem is that this treasure is really distant, like other half of the world kind of distant, and really hard to obtain.
>The drinker will feel the urge to gather certain ingredients to brew a certain kind of potion, he also gains the entire knowledge on how to prepare it but, he doesn't know what the potion will do. If not stopped the character will create the potion effortlessly. This new potion will make you repeat this process if drinked.