This is exploration:
DM say: You walked into a triangular room with a wooden door on each side. The floor is made of sandstone, stained like a bag of Skittles - all the colors of the rainbow. Spread throughout room are six steel domes, each with a steel handle.
DM's notes: Beneath each dome is a vat of liquid.
Dome A: Purple, bubbling goo. Dries after 1 minute. Acts like a mixture between glue and concrete. DO NOT DRINK.
Dome B: Silver. Smells like lavender. Anything placed inside the vat or in contact with the liquid. shrinks to 1 tenth its size after 1 minute. This is not a curse or magical enchantment. It is a physical transformation.
Dome C: Shamrock green. Smells like springtime. Any creature placed in this vat or who imbues its liquid is reincarnated, as per the spell. Even if it is still alive.
Dome D: Black as black can be. Because it's a terrifying black ooze monster. And it's hungry.
Dome E: Orange. Smells like ogre. If imbued, causes the character to sprout a second head. The head has the opposite alignment as the original character. This is not a curse or magical enchantment. It is a physical transformation. (Imagine spiking someone's wine with this).
Dome F: Yellow, fizzing. This is a very strong acid. Shatters glass. Eats away organic material. Will eventually destroy metal.
Chaos ensures as characters try to discover the secrets of the vats and decide how to best use that knowledge.
That's what I consider wildcard exploration, and I don't actually consider it to be a great example of how to make exploration interesting.
It only has variety if the players can actually figure out what it does. If they try a few things and nothing works, they may just decide it's a red herring or not worth their time.
Contrast that with a tree made of pure crystal that the PCs come across in their wanderings. They may or may not successfully deduce anything about it, but are nonetheless left with a bit of wonder about it. What is it? How did it get there? It's inherently more distinctive than a vat of liquid.
It again only has purpose if the players can and do figure out what the liquids do. Even then, it's more likely that it's a purpose that the players need to devise on their own (if any).
Compare that with coming across a forgotten back door to the dungeon you were trekking to. This has an obvious purpose. It's possible that the players may not make the connection, and might assume it is a different dungeon, but they certainly have the capacity to at least make an educated guess as to its purpose.
Lastly, the challenge of the vats is in experimenting to figure out what the liquids do (without doing permanently harm to your character). If the players decide that it isn't worth their time, there is no challenge. All they've given up is finding out what the liquids do (which for all they know could be water with food coloring).
By comparison, a river that needs to be crossed in order to reach the dungeon is a real challenge with plenty of options. They could try to swim across, look for a shallow ford, or even lash together a makeshift raft. While they could certainly turn around and go home, that isn't very likely.
I'm not suggesting that wildcard exploration is bad. I've played plenty of adventures which featured that sort of thing and it can lead to great fun and surprising hijinks. However, it can also end in the PCs walking away in frustration, with the players wondering why you wasted everyone's time.
While it's certainly an example of exploration, I don't really consider it to be a great case of exploration. IMO, it's something you don't want to overuse.