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. 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 House. Garbage Day cannot be Tuesday or Wednesday, since the unicycle was already in the dumpster. Gary had found the unicycle and brought it back to SpongeBob. SpongeBob is also seen riding his unicycle in the later episodes "No Free Rides" and "Born to Be Wild." 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 seen his unicycle in "Choir Boys." In the episode, he appears riding it next to Squidward's bicycle and asks Squidward if he is good enough to join the Men's Chorus.
He is seen riding it in "Karate Star." It has a wave-blowing like wheel and pedals onto it. In the episode, Patrick chops SpongeBob's Unicycle.
In "Traffic," he is seen using it and due to traffic, he trips out of it.
SpongeBob is also seen riding his unicycle to school in "Demolition Doofus." In the episode, the unicycle is red instead of blue.
It appears in "Sandy's Nutmare" when SpongeBob is using it to give the Bikini Bottomites some of Sandy's Nutty Butter, but is red again.