I usually give a nod and smile. I love encountering other runners who are out having as much fun as I and you can tell by the grin you get back. Some days when in the "zone" I will be slow to acknowledge another runner but if they make the effort I always respond with a grin and wave. I encountered an elderly man once who was out for a walk and he made a big point of acknowledging me and it made me feel great to be a part of such a nice communtiy of people. It definitely puts a spring in my step when there are other enthusiasitc runners/walkers.
I like to run evenings and I fiind it funny now that the nice weather is here how many people are out (several!) where were they on the grey, rainy, windy, snowy, frozen, pelting rain evenings of winter? I was the only crazy one out there then and really could have used a nod or smile 