Cream has several cameos in Sonic Adventure DX. She flies by after Sonic and Tails clear Casinopolis, she can seen by Big in the same area, and Amy spots her flying by the train station at Station Square.
In Sonic Adventure and Sonic Adventure DX, there is an unused dragon boss with two heads.
Contributed by SonicManEXE
There are various unused voice clips, all belonging to Tikal. One of these clips are instructions on how to transform into Super Sonic. As such, it was probably a dropped feature.
Contributed by RayMonkay
Sonic Adventure is the first Sonic game to feature downloadable content, as the Dreamcast was the first ever video game console to have online capabilities out of the box. DLC ranged from celebratory holiday decorations in the Adventure Fields and a few levels, such as on Christmas and Halloween, to national ranking contests and the addition of the Chao Black Market.
Jon st. John, the voice of Duke in the Duke Nukem games, also voiced Big the Cat for several Sonic games starting with Sonic Adventure.
Contributed by Skarro
An image of some staff members from Sonic Team, and a picture of a blue spined hedgehog, can be found in the game's data.
Dred Foxx, who voices Parappa in Parappa the Rapper, rapped the lyrics for "Unknown from M.E." (Knuckles' Theme).