Encountering other animals on a walk

What are the rules for this? Usually it's not an issue, but I had the scare of my life when a G. Shep. came along at the park the other day, off-leash.

My dog usually does not have issues with other dogs, and wants to play. It worked out well in this instance because the owner ran over to grab the other dog by hand, but I prefer not to avoid the park because it's such a good space for her to run and play. (I do not let her go off-leash in public, ever.)

It's really only a problem when we encounter an off-leash unexpectedly because most owners can size-up the situation and do what needs to be done to control matters.

What do you do when you have yours on-leash and the loner is suddenly upon you? I'm concerned because I can't really rehearse this scenario with the dog. I don't want a dog that doesn't defend herself if necessary, but I don't want her to engage if avoidable. I do what I can to avoid dogs we aren't familiar with, even if they are on a leash.