Riffing off shriekers, you could include a fungus that creates some sort of effect when touched. For example, I once used mushrooms that released a gout of steamy fog when subjected to heat (in an area PCs likely to encounter fire-based attacks). Clouds of hallucinogenic, sneezing, fear-inducing, etc, spores; or a spurt of slick, greasy slime could be effects that could compound the hazards of nearby terrain.
Yellow mold puffballs, or other alternative forms of known varieties of dangerous fungi.
Sentient slime mold (reskinned slug/worm or something; which communicate via wet whistles or telepathy?)
I once used a mythic "Amethyst Tree" which ultimately proved to be a vast and ancient semi-sentient violet fungus that communicated prophecies in dreams.
The gills of a large overhanging toadstool or bracket fungus could be the home of a swarm of bats, spiders, stirges or something.
Invisible phantom fungus is fun. I've used them as beasts of burden and food for grimlocks. In 3e, they remained invisible after death... which I assumed also meant that the grimlocks' favorite meal -- phantom fungus fillets -- were also invisible.
A witch might fly around in an inverted toadstool, perhaps scattering poisonous spores upon foes.
