DS 6 (almost7) loves the snap circuits (he's also a lego fanatic). He got the snap circuits about a year ago and still loves pulling them out. It is nice to have an adult work with him from time to time to talk with him why things in series work different from in parallel and how what he is seeing is an example of what he's read about (light energy, movement, vibration causing the noises etc), but he loves to just play and explore them on his own.

This year he's getting a beginner mindstorms set. At 6 he'll need help with it for sure, but he's so interested in robotics that I think it's time to get his feet wet (and ours too)

I also find that Magnatiles is an excellent toy. All three of my kids love it. Ages 3,4,6. It is like 3-D tanagrams with all sorts of fun geometric shapes. Not cheap, but well worth it. http://www.amazon.com/Magna-Tiles-Translucent-Colors-100-pieces/dp/B000CBSNRY/ref=pd_sim_t_2

Also don't forget a chess set. DS loves pulling that out over and over too - if he doesn't know how, no stress chess is a nice way to start.