In Bouler County, you should be asking for a Diversion referral. A criminal defense attorney with experience should be able to negotiate a very favorable plea disposition in such a case -- based on what you've told us in your note.
Best of luck to you.
It is worth noting htat the theft statute requires that the government prove you had the intent to permanently deprive the rightful owner of possession fo the bicycle. If they cannot prove that, they cannot prove their case beyond a resonable doubt. Make sure you let your attorney know that you had no intent. While that may not be completely believed by the prosecutor, it is something that may be considered.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.
Mr. Deasy provided you with good advice. Theft does require that you have the specific intent of taking the bike and permanently depriving the owner of the bike. A simple joy ride does not necessarily mean theft. You went on a joy ride. If you are a juvenile, you will likely be run through diversion and be required to do public service hours, take a theft class, and stay free of charges.
The information provided in this answer does not create an attorney-client relationship. If you are interested in his legal services, feel free to call Chris at (303) 409-7635 at his law office in the Denver Tech Center. All initial consultations are free of charge.
The fact that the bike was unlocked is not a defense; However, as my colleagues have said, your lack of intent to steal is. It really comes down to whether you have had any prior criminal contacts--obviously, the statement of a person who has a history of thefts that he intended to return the bike is less likely to be credited by the prosecutor and judge. Typical pleas in such cases range from a "deferred" (where the case is ultimately dismissed if you stay out of trouble and comply with any other conditions) to jail (if you have a criminal record). Consult a lawyer, and Good Luck.