Would bringing visitors to one's university (to give talks and collaborate on research) be considered a synergistic activity? I think one could certainly make a case that it is, and I'm curious what others on this forum think. My last NSF proposal was submitted before I'd invited any visitors, but I might consider listing this sort of activity the next time around (which is frighteningly soon...). In my opinion, bringing in speakers is quite important as a synergistic activity, especially when the speakers give talks that are geared towards a broader audience than research experts in one area. It is excellent, for example, as a way to expose graduate students to current research.