There is no way for the cyclist to tell whether or not the guy driving is being friendly or mean, so don't honk. I just always give the rider plenty of room and pass with reasonable speed for the road.

I have "blocked" for cyclists on several occassions. For example if coming to an intersection I'll slow down way in advance of the cross street so that the rider can take the lane if needed. I'll do this especially if there is traffic behind me.

The "thumbs up" or a wave is a good greeting. In Texas you can get "Share the Road" license plates. Riders here can spot those pretty easily.