To celebrate the release of Sonic Adventure, Yuji Naka (former head of Sonic Team) joined a line of fans in Tokyo's Akihabara district waiting to purchase the game. Accompanying him were 4 people dressed as Sonic, Tails, Knuckles, and Amy.

Windy Valley went through major visual changes during the game's development. In the intro sequence, Tails and Big can be seen standing in an early prototype of this stage. In fact, Big isn't even able to access Windy Valley in the final game.

In the first Japanese release for the Dreamcast, a mechanical cowgirl billboard could be seen within Casinopolis. It would moan whenever the player touched it and was removed in international copies of the game for obvious reasons. This version however was also toned down from the Sonic Adventure Auto Demo prototype, where the girl was a fully fleshed playboy-style bunny. [Image credit: evilhamwizard]

The hidden black Chao egg found within the Egg Carrier has a unique texture in the original version for the Dreamcast. It was blue and black with pink spots. This was probably so that it would fit in better with its surroundings. This was changed in later releases of the game to solid black. Opening it will hatch a regular black Chao with seemingly no special qualities, however.

The Dreamcast disk has a PC-readable folder named "extras" that contains nine different wallpapers based on the playable characters, plus one of a Chao. Some of these pictures would later be used in the gallery in Sonic Mega Collection's Extras menu.

In the Dreamcast version of Sonic Adventure, you can explore under the deep water in the Egg Carrier Chao Garden. Under the water lies a secret ring far out of reach. This was removed in the DX update, because the depth of the water was reduced, probably to prevent Chao from swimming out of reach and getting stuck in the terrain.

Knuckles' theme song, Unknown from M.E., incorrectly states that he is a porcupine.

The song "Twinkle Cart", used when Sonic first enters the Twinkle Park stage during his story, is a remade version of Panic Puppet Act 1 from Sonic 3D Blast (from the Genesis version). The composer of both is Jun Senoue, the guitarist and keyboardist who is a key member of the band Crush 40.

The background music in the second area of Windy Valley, The Air, is an arrangement of the music from Green Grove Zone in the Sega Genesis version of Sonic 3D Blast.

Some of the textures used for E-105 ζ are actually pictures of Dreamcasts.

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.

Perfect Chaos' appearance in Sonic Generations is different to its appearance in Sonic Adventure. In that game, it was a creature made entirely out of water, whereas in Sonic Generations, it has scaly, reptile-like skin. According to an interview with Takashi Iizuka in the Prima Strategy Guide for Sonic Generations, Perfect Chaos' 2011 design is how the team had originally planned the monster; it had had to be changed in Sonic Adventure due to the technical restraints of the Dreamcast.

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.

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.

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).

Just before Sonic transforms into Super Sonic near the end of the game, the surviving crowd in Station Square can be heard in the background chanting Sonic's name to encourage him. This crowd recording is actually comprised of over one thousand attendants in the audience for the official reveal of the game at the Tokyo International Forum on August 22, 1998. The chant was led by special guest Hiroshi Fujioka portraying Segata Sanshiro, the Japanese advertising mascot for the Sega Dreamcast's predecessor the Sega Saturn, and recorded by Sega to use in the game.

In Final Egg, two large stasis tubes can be found. One holds Metal Sonic, while the other holds an unknown Sonic-based robot, later referred to as Mecha Sonic Mark III by Ian Flynn. This robot has no importance in the story, however, it seems to be based heavily on the design of Mechagodzilla from the "Godzilla" franchise's Shōwa era, due to many similar parts and possible textures from the robot. This also seems to be a reference to Mecha Sonic Mark I, also known as Silver Sonic from Sonic the Hedgehog 2, whose concept art also shows similarities to Mechagodzilla.

In Speed Highway, several billboards can be found depicting an unknown female character. In 2020, Satoshi Okano, a former designer at Sega who worked on Sonic Adventure, revealed that this was a scrapped spider character he designed with the prototype name "Spider Girl", and posted an old sketch of the character alongside a sketch of another scrapped character, a bat named "Fly the Vampire". In 2023, Okano posted an inked version of the original Spider Girl sketch from the same time period alongside several inked sketches from the development of Samba de Amigo. Spider Girl's appearance on the billboards in Sonic Adventure suggests that the character lasted far enough into the game's development to receive a 3D model, but was ultimately scrapped for unknown reasons, and this 3D model has yet to surface on its own. It's unknown what role either Spider Girl or Fly were intended to have, though fans have noted that Spider Girl has some similarities to Rouge the Bat from Sonic Adventure 2, namely that both characters have large bosoms and prominent heart-based/romantic imagery.