What other non dive related things should I do and experience while I am there?
In addition to what has been mentioned stroll through the Mercado Municipal (municipal market) in town and the shops around it - Great goods at a fraction of the price in the tourist zone near the water. See the wild east side with your car (or the flat calm east side if the port on the west side is closed due to a Norte and you can't go diving). I'd probably hold off on renting a car until you are there and keeping an eye on Wind Guru Cozumel. If you see winds forecast to be blowing out of the north at 20+ mph the port will probably be closed so no diving... That's when I'd rent the car a day or 2 out from that nasty forecast. Other than seeing the east side and the ruins, I think you'll have little use for the car since you are staying in town. Have google maps on your smart phone and download the offline map for Cozumel so you can find everything without having to pay for international data if you provider charges for it.
Should I make a trip to the mainland?
I personally wouldn't. Far better diving in Cozumel and while there are Cenotes they are COLD compared to the ocean and there really isn't much to see in them in my opinion. Some people love them, others think they are waste of time.
Should I use Pesos or US dollars for my day to day purchasing?
Pesos withdrawn from any bank ATM machine via a debit card at a bank or the Mega. Depending on the bank, some will ask you to accept a BS exchange rate that they post. Click on DENY and your transaction will go through based on your legitimate Canadian bank's exchange rate being offered that day based on the spot exchange rate. Why would I accept this Mexican bank's 17:1 exchange rate when my bank is exchanging at 20:1? All the Mexican bank is doing is handling the transaction and the collect a small fee of a few dollars. This scam is showing up in tourist areas everywhere where people don't know better and the Mexican bank pockets the difference between what my US bank offers and what one may unknowingly agree to by pressing that "Accept" button instead of the "Deny" button.
Any don't miss it food dishes I should try?
Take your pick off the menus of these places. All in town or cheap taxi ride if you don't feel like walking it. When I'm grabbing taxi's to travel about the town and close zones I just get in, tell him the establishment I want to go to and hand him 50 pesos. That's about 2x the official rate and I'm being generous. Locals DO NOT tip taxi drivers. Below is a link to the most recent taxi fare schedule I could find.
You'll probably be in Zone1 in your condo and as far as you'd be electing to take cabs around San Miguel is probably into Zone 2 or 4 so the official rate to zone's 2 and 4 is 27 pesos. Obviously handing him 50 pesos is a nice fare. $2.50 US to get where I want to go quick and easy is a no-brainer. This is why I say renting a car is nice but only if you are going to venture far from town. If not, it's far cheaper to cab it around.
Taxi rates
Los Otates (now no name but everyone knows where it is)
Los Otates, Cozumel - Restaurant Reviews, Photos & Phone Number - TripAdvisor
El Moro
El Moro, Cozumel - Restaurant Reviews, Photos & Phone Number - TripAdvisor
Los Sera's
Los Sera's, Cozumel - Restaurant Reviews, Photos & Phone Number - TripAdvisor
Benitos - Italian with brick oven pizza - Try the spicy shrimp pizza.
Benitos, Cozumel - Menu, Prices & Restaurant Reviews - TripAdvisor
Burritos Gordidos or Crazy King Burrito (they are pretty much beside each other).
Burritos Gorditos, Cozumel - Menu, Prices & Restaurant Reviews - TripAdvisor
CRAZY KING Burrito, Cozumel - Restaurant Reviews, Photos & Phone Number - TripAdvisor
Chilangos Taqueria - CLASSIC Huaraches
Chilangos Taqueria, Cozumel - Restaurant Reviews, Photos & Phone Number - TripAdvisor
El Billy Asados Al Carbon - BBQ. BBQ Sampler. Staying in a condo in town you'll have enough food to take back with you for 2 more meals (or to bring on the dive boat and share with everyone the next day)
El Billy Asados Al Carbon, Cozumel - Restaurant Reviews, Photos & Phone Number - TripAdvisor
Casa Denis - For a dinner in town square. 1 order of the the Fajitas Mixtas (Beef, Chicken & Shrimp) is enough for 2. Drinks aren't "cheap" there.
Casa Denis, Cozumel - Menu, Prices & Restaurant Reviews - TripAdvisor
Casa Cuzamil - Cheap comida del dia before 6PM. Usually has 5-6 entree selections for the Comida del Dia. I go with the chili relleno.
Casa Cuzamil, Cozumel - Restaurant Reviews, Photos & Phone Number - TripAdvisor
Burger King - When you get sick of Mexican food and Mexican beef (which tastes a bit different from the beef you may be accustomed to in Canada - I know it tastes a bit different from US beef).
The above is my "must eat" list but others have their own and the only reason I can't comment on their recommendations is I haven't had the opportunity to eat at them yet.
Sounds like you are doing everything right... Staying in town, diving with quality ops (3P's is my op as well), and are asking all the right questions. Glad to see you are interested in dining at establishments that offer far better food and lower prices than can be found in the tourist zone. The one problem with Coz is you could visit annually for a few weeks for the rest of your life and not run out of existing and new restaurants to try (or dive ops for that matter). The other prob with Coz is it is going to ruin you for many other dive destinations. The quality of the diving, the low cost of diving and topside accommodations in town, the ease of getting there... We dived Coz for the first time 17? years ago now. Been back every year ever since. We'll squeeze in other dive destinations but those are costly and far away. Coz is going to interfere with your sense of adventure and desire to dive new places because you'll want to go back over and over and over.