Well I think the previous poster got most of it for must do's. Not hard plenty of time to do all that.
I was born and raised, but moved away 10 years ago.
I'll just add:
Food:
Go to Alycia's and get some perogies
Sightseeing:
Spend some time in Old Market Square area
Check out the Fort Garry Hotel for brunch on Sunday (or do they have it every day?) not sure. Then take a brief elevator ride to the top to see the skyline... it's a free ride to check out the view (hidden tip).
that's all I can think of that hasn't been said... I am a little out of the loop these days about the place
Depending on when you go also, there's sometimes lots of festivals and things going on. In the summer the city can get quite empty on weekends as everyone drives out of town on Friday for mass exoduses to lake cottages and beaches. Depending on when you go, check out the Fringe Festival, Folklorama (that's in August), and the Folk Festival.
PS...
wear a lot of mosquito repellent