Reward: 200 Renown, 2000 gold.
First time you visit Bowerstone South, you'll see a little girl who will tell you to follow her. You will meet her mother, taking care of a seriously diseased boy. The Bowerstone Quay witch can prepare a cure for him, but she needs four blue mushrooms as the ingredients.
1) You'll encounter two rather strange traders in Lookout Point, who'll be talking about the mushroom they lost. Talk to them a few times and they'll tell you one of them threw it into the pool near the Heroes' Guild Demon Door. Go there and fish it out.
2) There's an even stranger woman in the Picnic Area. She will ask you to make her laugh. Use some funny expressions on her, such as turkish dancing or so. After a few tries, you'll be rewarded with the second mushroom.
3) Go to Guild Woods and talk to the man standing in there. He'll ask you to deliver a love letter to a girl in Oakvale. You can do as he wants (good solution) or tell the girl you wrote the letter yourself (evil solution) and either way show him the reply. A mushroom will be your reward.
4) You can buy the fourth mushroom from a trader in Barrow Fields. It costs quite a lot for something like that (1500).
Go back to the witch and give her the ingredients. It seems she had the cure all the time, and she only needed the psychotropic mushrooms for.. well, tests. LOL :-)