1. Do you need a job this summer?

“The Ontario government — and its partners — offer many programs to help students find summer jobs. You need to apply for most programs. In some cases, the government can connect you to people who can help you with your job search.”
Visit https://www.ontario.ca/page/summer-jobs-students to learn more.
Also visit https://www.gojobs.gov.on.ca/Pages/SEP.aspx
Or the Canadian site: https://www.canada.ca/en/services/jobs/opportunities/student.html

2. Of real interest:

You may also want to visit www.ontario.ca/summercompany
If you are between 15 and 29 years old, for “business training, mentoring and awards of up to $3,000 to start and run their own summer businesses”.

3. Other sources suggest various urls to visit:

https://www.macleans.ca/work/jobs/10-websites-with-summer-jobs-for-students/
https://www.thestar.com/business/personal_finance/2011/05/12/7_good_websites_to_help_find_a_summer_job.html
https://torontopubliclibrary.typepad.com/teens/jobs.html
https://www.linkedin.com/jobs/summer-intern-jobs?country=ca
https://www.glassdoor.ca/Job/ontario-summer-student-jobs-SRCH_IL.0,7_IS4080_KO8,22.htm

4. Perhaps you are interested in a paid internship:

https://www.looksharp.com/s/paid-internships/toronto-on
https://ca.indeed.com/Paid-Internship-jobs-in-Toronto,-ON

5. Upwork.com site – bid on jobs OR offer your services
https://www.upwork.com

6. Other more traditional ideas:

Word of mouth
Networking
Calling local businesses
Knocking on doors
Raffling off your services (e.g. painting, writing, admin) to the highest bidder
Approaching laboratory and other scientific directors
University employment offices and web sites
Want ads
Summer camps
Banks, drug stores, retail stores, big box stores
Many big corporations – call the HR departments
Depending on your age, Uber or Lyft or Airbnb

If you happen to be Down Under for the summer, take a look at https://www.airtasker.com/jobs/ Australia’s largest Job marketplace.