In story mode, Sonic is the only playable character, but in a normal race, every character is valid. The course takes place at daytime, in a desert. The start/finish line is in a colosseum, surrounded by people watching. Right before the end, there is a giant bow that shoots the racer back to the colosseum. There are many ruins throughout the course, some of them being obstacles that can be hit by a power-type character, or must be avoided by any other character that's not strong enough to break through.
As the name replies, Sand Ruins takes place in the middle of a desert temple. The temple is in ruins and sand flows just about everywhere. Few ramps are in the area and some dash boards, as well.