The Goonies/WMG

Annie the Mermaid from the video game sequel is, in fact, Sloth.
Some randomly visiting deity; possibly Haruhi Suzumiya, figured that there needed to be a love interest in the story; and that Sloth was the wrong gender and kinda ugly. One POOF later, and ta-dah!

Mikey seems to know more about One-Eyed Willy than he's telling.
Mikey tells One-Eyed Willy he's the "first Goonie". Could it mean One-Eyed Willy is his ancestor? Was the surviving pirate who escaped the massacre with the map and the doubloon Willy's son?

Mikey is the reincarnation of One-Eyed Willy
An elaboration of the above theory; he seems to have an almost intuitive knowledge of One-Eyed Willy's treasure: he is able to find what professional treasure hunters couldn't, he successfully avoids the traps (especially the final one), and he even has a similar love of Rube Goldberg-esque devices. And in a moment that made it to the novelization but not the film, Mikey finds a primitive version of an asthma inhaler next to One-Eyed Willy's skeleton, just like the one he's been taking hits off of the whole movie.

Chester Copperpot is Mikey's grandfather or other relative
Chester Copperpot was one of the only people to know about the treasure and look for it (even though he failed). Mikey finds a map and a metal or stone key in his attic. These items, but especially the skull-shaped key, are probably the only ones of their kind, so in order for Copperpot to get as far as he did, he would have needed at least the map. However, he didn't bring the map with him because he either memorized it (he was a dedicated old coot) or he found the shortcut through the well. Mikey has the map and the key because Copperpot is his ancestor, and his dad inherited it when Chester never came back. This would also explain the newspaper article in the attic.
 * Though he couldn't be his paternal grandfather, since his last name obviously isn't Copperpot. Its very unlikely that's Mikey's mother's maiden name, because its not a very common surname, unless his mother's side of the family changed his name. On the other hand, Mikey's dad was probably an only child and didn't even know Chester was his dad.

The treasure map clues conveniently rhyme in English because they were written in English
The Conveniently Precise Translation is possible because One-Eyed Willy (not exactly a Spanish-sounding name) wrote the clues in English originally, and then translated them into Spanish as an extra layer of obfuscation. Discovering that the English translation can easily be made to rhyme is a clue to the person using the map that they have the correct wording.
 * I call this canon. It makes sense as yet one more way Willy was able to mislead and fool anyone searching for the treasure, and it also fits both his name not being Spanish (unless Willy were a translation of a diminutive for the Spanish version of William) and the fact that few Spanish pirates would have been plying the waters off Astoria. (If anywhere, they would have been down near Los Angeles or Mexico.) As to how he knew Spanish--interaction with said Spanish pirates, and the Spanish authorities in California. The intelligence required to build his devices already shows him to be a well-educated man (and a number of pirates were).

One-eyed Willy had that name long before losing an eye.
Not that they'd mention or know it in the move, if you know what I mean.
 * He didn't lose an eye, he literally did not have one. He had no left eye socket either.

Stef is A Vampire
When Mouth pulls the mirror prank on Stef, there was no reflection, but she still reacted badly to it. Maybe she didn't want anyone to know.