Good timing, I was just in SLO this weekend! 

From what I saw of the downtown area it seemed pretty flat, especially compared to SF. Sidewalks are in good condition if that helps at all.

By the way if you're in the mood for some coffee, definitely stop by  Bello Mundo

It's a nice little boutique place with amazing espresso drinks and great pastries to boot.

Heels, Flats, Tennis Shoes, whatever, just as long as they're comfortable.  We don't have very many taxis, and renting a car here is pretty expensive, so we walk, bike, take the bus (I don't recommend it), or take your own car...  We mostly walk/bike, especially when you get near California Polytechnic State University, San Luis Obispo.  It's mostly flat land, so heels are fine, but I'm recommend low heels and flats, just for comfort and maneuverability.  Hope you have fun!

I think reasonable heels should be fine.  Campus is slightly hilly, but nothing too treacherous.  If you've been to UC Berkeley before (which I take it you probably have since you're coming from SF!), the terrains of the campuses are very similar.  Lots of grass and greenery, but also paved roads.  There are hills, but not crazy ones, and there are stairs.  Just a FYI, a lot of people on campus actually wear flip flops during this time of year.

As far as downtown goes, totally flat!  Very clean and well-paved, so heels would be fine there, too.

Hope that helps!