I'm pleasantly surprised by what I've been able to do with my LEGO Mindstorms kits.

While it's not a very powerful computer, the Mindstorms controller does turn out to be well matched to the domain (controlling robots) - and the graphical NXT-G programming environment is outstanding.

The new-style technics beams and parts are awesome - the 'bots very seldom come apart, and when they do, they're typically doing that rather than breaking the plastic parts.  That's a desirable balance.

(click-right and "view image" to see the full-size version)