Joni's backpack is different in this game than in others. Instead of opening with a vertical zipper on the back, it has a latch on the bottom and opens similar to a suitcase.
The ClueFinders wear different clothes than in other games they appear in. By 5th Grade Adventures and onwards, they revert to their original clothes they had on in 3rd Grade Adventures.
The dog Socrates makes his first and only appearance in this game, but in later games he's mentioned in the character bios despite not tagging along for more adventures after the events of this game.