ETA: Thanks for the advice guys! I decided to go with the dress, black tights (because it is cold in Toronto!), long black boots and a longish cardigan that I can take off for photos!
At a rented out bar with other 25-year-olds? I have no idea, I always stay in NYE. I have this dress from H&M, but I feel like it's too nice for pre-drinking and going to some possibly dingy bar to drink beers.
Any ideas? I'm deep in the grad school dress code of jeans and sweaters.