You can also to to jail for selling chewing gum. And you are legally required to flush a public toilet after using it.

Only for quantities consistent with trafficking, meaning a minimum of 500 grams. You won't be hanged for a joint.

You may, however, be sent to the Drug Rehabilitation Centre, which is co-located with and effectively a part of Changi Prison, and about as pleasant. Most first-time offenders get away with a probation scheme called the Enhanced Direct Supervision Order though.