In my world non-original:
1.) Trees with partially metallic leaves
2.) Rivers that "sing" with Elvin melodies
3.) Boulders and Mountain sides that are "spongy"
4.) Rivers of blood. Literally.
5.) Cities and towns where the houses and furniture are now eatable. I mean edible. I mean "you can eat them.."
6.) Flying Cities hidden in the clouds
7.) Cities that are "cursed" and moving through time in large chunking leaps.
8.) A glass mountain run through with threads of precious metals.
9.) Trees and forests that need blood instead of water.
10.) A world thats flat..
In my world, original (inso-far as my conscious mind knows)
1.) A country blocked off by a wall of fire 300 feet high and 150' wide which only the "faithful" of a Greater Diety can walk through unharmed.
2.) A great plateau 1500 miles long, in some places 3 miles high, which seperates the lands and has caused its people to designate "East vs West"
3.) An inward facing 400 feet high ring of black shale (and schist) stone positioned like a "/" facing inward into a desert of glass (sand on top, thick layers underneath) which contains the trapped soul of a dark god. Implosions as far as I know still make an "outward" blast ring; this one didn't.
4.) A simple plains surrounding 4 large black pyramids. One day a year the fields of this plains are filled with jack rabbits that at a certain time a day call out one word "Niibitz"...
5.) A tree made of black magical steel, which is alive but has no leaves or seeds.
6.) A moon made from magical ice.
7.) Huge deserts made up from the steel, stone, and glass dust of a city that once stretched over 100 miles.
8.) A country that where winter lasts 1/2 of the year, summer the other half, and whose inhabitants change to match the mood of the season (including the ruler!)
9.) Seas made of glass.
10.) Events: 1 day out of the year when all of the oceans, seas, lakes, rivers, streams, etc turn to solid ice for 1 hour.
hope these give ya ideas.
Edit: note - for those asking about harmful side effects from any of these "unnatural natural occurences" most of the time you can say "none". Physics, biology, etc. falling a bit off center as the effects cause is "magical". However in the case of the "rockblast/dark god" area - PCs and creatures moving through the area suffer heat & electrical damage (d6 using the DMG guidelines for environmental affects and increasing accordingly) due to radiation, etc. Also they suffer magical mutation randomness (borrowed from an old Gamma World (tm) book)