We have been in Orillia two times. Business friends invited us the first time and we enjoyed beautiful weather. Especially, the waterfront was an attraction for us.
Orillia is blessed with a beautiful location on two lakes - Lake Simcoe and Lake Couchiching.
Elizabeth, my wife, did some window shopping in downtown, but we did not go to the casino. Casino Rama is a well known attraction. It is open twenty for hours, but I did not want to lose the little money which was left in our travel budget.
Unfortunately, we lost all photos during a break in.
During our second visit I wanted to take pictures, but as you can see we had heavy rain and we could not even leave the car.

Check out the actual program of the Orillia Opera House. I never saw that they announced an opera, but other interesting events, such as the Vinyl Cafe.

The Orillia Museum of Art & History is another place worth to visit. Just stroll through downtown and you should find it.
Find a nice place to stay and explore the surroundings. Nature is always a main attraction.