If you are on an official break then sure you can work full time as many hours as you want. Cannot add anything on working with a taxi as would expect you need some licensing and insurance to work.
And you would have to register with the CRA to get a GST/HST number. The taxi industry falls under mandatory registration, meaning the small supplier threshold does not apply. Failure to register does not alleviate the requirement to collect and remit. Failure to register, you would still be on the hook for the tax. If your SIN starts with a 9, you would fall under the non resident registration procedure. Call the CRA, they will advise you of the office that processes the registrations for your country of origin
Typically registrations for non residents are more stringent, since there are crown funds involved
Having a SIN is MANDATORY for registration.