I've had to run all over the place while pumping several times.
First, I always ask if there's a place, office, or somewhere I can pump. It never hurts to ask, and a lot of times, someone can fix you up with something, especially if you are able to ask before hand.
When I don't really like what's offered (Like the couple of times someone suggested I use the bathroom, for example, which I find yicky) I will go out and pump in my car (w/ the A/c on) w/ a battery pack or w/ the adapter. If you really don't want other people to be able to see you much, you can use one of those wind-shield sun-shade things, and that really lowers visibility. Also it keeps your car cooler, so, double bonus!
If you need to build up a stash, the best time is early in the morning before everyone wakes up.
I found that some days, it was actually quicker/easier to use a hand-pump on one boob for an ounce or three than it was to get out the whole set-up and clean it all up after. So that may be something you might think about. I used both the Lansinoh hand pump, and the Avent Isis hand pump. Both have that funny silicone "massage-y" thing that supposedly can help you let down faster/easier. I do like it. The Avent comes with a set that makes it really easy to transport and keep from getting messy even after you pump, but only fits the non-standard bottle sizes.