Mushroom is randomly found in the woods: +0
Mushroom is randomly found in cave: +2
Mushroom is being cultivated by some sort of monster/non-human: +7
Mushroom is part of a fairy ring: +10
Roll a d20 and add a modifier if applicable!
1-4: Agh, ptthghh, Ptui - This mushroom isn't really edible. Either it's too hard to chew (1-2) or it just plain tastes nasty (3-6) with no real other effects.
5-8: Meh - The mushroom is edible without really tasting of anything or any other strange effects. If enough of them are collected, they'll act as rations from a caloric standpoint, although they'll go bad unless dried and persistent reliance on them as a food source will lead to malnutrition (unless you are a goblin).
9: Kind of Tasty Actually - These taste pretty good and will taste even better if cooked in a soup, roasted or spread on toast. A pound of these will sell for 5gp (1-2), 10gp (3-4) or 30gp(6) to any inn/cook with high-end enough customers.
10-13: Poison! - Most of these mushrooms (1-3) will cause the character to vomit up everything in their stomach and expel everything below that, extremely violently, unless they Save vs. Poison (in which case they just yak up the mushroom itself). This will continue until the player makes a Save vs. Poison -- they are allowed one every 12 hours after the incident with a cumulative +1 bonus to the roll. Until the save is made the character is unable to eat and can only imbibe small amounts of clean water. They will be weak and unable to do much more than lie down and groan.
Some (4-5) will do more debilitating damage to your insides. Save vs. Poison -- a success means that you spit out/throw up the mushroom and only take 1D6 temporary damage to your Con. A successful further save, rolled weekly, will regain a single point. If the player fails the save, the damage is permanent and can only be regained through various magical means.
Aaaaaand some'll kill you (6). Save vs. Poison or some important organ gets liquified. If you make it, treat it the same as the moderately poisonous mushrooms.
14-16: Confusion - As per the spell as cast by a 5th level caster.
17-18: Prophetic Visions - The character is becomes a viewer to some sort of important event. Whether it's an invading army, witnessing the summoning of a powerful demon, something abstract and extra-planar, it's extremely vivid and unsettling. The character will have no idea whether it has already happened, is currently happening or is going to happen. The character will be out of it for 3-5 hours, although only the relatively brief vision will be memorable.
19: Magic Mushroom - Randomly pick a first-level magic-user spell, targeting the eater unless it's an offensive spell, in which case the eater knows the spell and has to cast it in the next three rounds. If you're feeling literary, skew the results to Enlarge/Shrink.
20: Spirit Quest - The character, either in their body or in the body of somebody else, finds themselves in another plane of existence. Maybe it's Fairyland, maybe it's Elysium, the Astral Plane, whatever. In any case, they're somewhere else entirely and there's some sort of mystical being chatting at them about what they need to do. Their regular body will appear to be in a coma to the rest of the party, although attempts to communicate with the character's soul/spirit will contact the character in their new existence. The rest of the party will experience the same result if they also eat the same mushrooms.
(If the roll is over 20, there is a 33% chance each that the result is either Prophetic Visions, Magic Mushroom or Spirit Quest. Roll/decide which would be more fun/would fit best with what you've got prepped.)