The unicycle is blue and somewhat tall. SpongeBob called it a "bike," which is technically incorrect.
SpongeBob first threw it into the dumpster at Mrs. Puff's Boating School, but later he was saddened when he did not have his bike to ride all the way back to the hospital to cheer up Mrs. Puff. Gary had found the unicycle and brought it back to SpongeBob. SpongeBob is also seen riding his unicycle again in "No Free Rides" and "Born to Be Wild."
He is lseen riding it in "Karate Star." It has a wave-blowing like wheel and pedals onto it. The unicycle can somehow ride fast because it takes less than three minutes to get to Mrs. Puff's Boating School, unless the Boating School is very close to SpongeBob's Pineapple. Garbage Day cannot be Tuesday or Wednesday, since the unicycle was already in the dumpster. Stephen Hillenburg probably (this is an opinion) changed the process of SpongeBob's transportation from unicycling to school in Season 1, to walking to school soon in Season 2 (right after "No Free Rides").
SpongeBob is also seen riding his unicycle to school in "Demolition Doofus." In the episode, the unicycle is red. It appears in "Sandy's Nutmare" when SpongeBob is using it to give the Bikini Bottomites some of Sandy's Nutter Butter, but is red again.