You’re a smart man to not visit Miami May-December. It was in the 90’s here today and with the humidity it felt like 95. Fok that. It’s cooling off a little tommorow.
Hmmmm good question. I take my sons mountain biking (yes we have great trails here) Jan-April. Biking on Virginia Key in Miami is a ton of fun.
You can do some fun people watching on Lincoln Rd in South Beach. If you’re renting a car you can park for a very reasonable rate in the multi story 17th St parking garage. Lots of good restaurants/gelato places on Lincoln Rd.
Zoo Miami is quite good. You can rent 4 wheel bicycles (that hold 2-4 people) and leisurely pedal your way around to see Silverback gorillas, African/Asian elephants and even feed friendly giraffes. Zoo Miami is about as west Miami as you can get.
If it was the year 2020 you could take your son to see Inter Miami play their inaugural soccer season.
Islamorada and Key Largo are only about a 45 min drive from Miami. Islamorada is a fun place I go once a year so I can only imagine that someone who lives in Toronto would think it’s a bit of paradise. Two restaurants in Islamorada are outstanding: Morada Bay (at sunset) and Lazy Days have delicious fresh food with outstanding ambiance.
Have fun. I’m available to meet for some bourbons at an adult establishment for an hour or two should you have the time for that.
I like to stay near Brickel area. It's away from the beach craziness (surprise - I hate the beach) and it's very accessable by train/metromover to the airport and close to many nice restaurants. A bit cheaper is "downtown" I believe. But IME that's a little further from restaurants/amenities. If you DO get a car and are in north beach take a drive up to Hollywood and take the boy to bet on the horses at Gulfstream Park! Fiver would be proud.