To me, Sonic Adventure is one of the greatest of Sonic's "adventures". And being that this is Sonic's first fully 3D game, you can be sure that there are a ton of early ideas, scrapped concepts, and unused objects. There are so many, in fact, that I couldn't even fit them all in a 24 minute video. So, feel free to check out the link below for even more info!
Enjoy!
Sources:
http://www.sonic-cult.org/dispgame.php?catid=3&gameid=4
https://tcrf.net/Sonic_Adventure_(Dreamcast)