How are level-up bonuses determined?

When the player level (not fever level or character's power-up level) increases, the player is rewarded with some items. I'm currently at level 35, and these have almost always been 15 red star rings. However, I've also infrequently gotten a Chao token, and some number of rings.

How are these bonuses determined? Are they randomized, or are they paired with a level... so that every player that hits level X will always get item Y? Is there any listing of all possible bonuses rewarded at level-up time?