u/Few_Pension_3059

Note: I'm not ranking them because it's a matter of personal preference. The best time window is 10PM to 2AM unless otherwise mentioned.

UC 4th floor washrooms with green walls. Basically it's on the side of the CUSA office there is a hallway. You will pass a fire extinguisher on your right as you are walking. At the end you will find the washrooms.
(If you are a science student) You can tap your card to access Herzberg after hours. I usually prefer 4th/5th floor because you will have time to react if the janitors are there. Usually they don't come on Saturday nights.
Tory 4th floor after hours. Just go there at 9:30PM and stay. Tory is closed after 10ish PM.
Accessible washrooms on the 3rd+ floor CTTC.
Tunnel-level Loeb classrooms/labs. A lot has happened there....
One/Some of the engineering buildings have breakout rooms.
St. Patrick's is usually open after-hours. There are two classrooms on the third floor I believe.
Washrooms on the 2nd floor of Teraanga Commons. If you're coming from the residence reception desk, walk towards the exit to Campus Ave but instead of taking the stairs down take a right then walk all the way to the end and then turn left.
One accessible washroom on the 1st floor of Teraanga Commons but if you're quiet enough.

If I think of other ones I'll post them.