This isn't as common anymore, but I used to sit by the lagoon at the university and just smell the lilies. It is my favorite scent in the world, and it served as a wonderful start to my day.
Like any city, and perhaps a bit more, Quito has some major pollution. With the dirt and smoke, the exhaust fumes of the buses are incredibly strong. Although my eyes don't get as red by the end of the day anymore, I still have dirty buggers when I blow my nose at the end of the day.
This happens more often in the south part of the city, but it definitely exists. It's a strong scent, which makes the source of it a mystery. I would also like to point out that public bathrooms cost money, with a bit more being charged for toilet paper. (Also, the urinal costs $0.10 whereas the toilet costs $0.15, which is clearly not fair.)
I have a weakness for bread, and passing the bakery is my kryptonite, particularly when freshly baked. I've scouted out several bakeries with delicious cinnamon rolls and other various breads, for $0.12 to $0.35 per piece.
I never realized how nice it is that tobacco is not allowed on the HPU campus. The plethora of students and faculty smoking outside buildings means that the scent is everywhere. I'm still not used to it, but it is less of a bother than initially.
With chicken and rice most nights, the smell of dinner cooking in the morning and then being reheated at night is wonderful. Even though I check to make sure it really is chicken (rather than cuy--guinea pig), I love the smell and the fact that my host mama makes it just for me each day.