So I know it isn't screen accurate but I was thinking it would be neat to put a small wrist watch sans strap into the range finder so when you put it down you can see the time. Helps you get to those convention events on time
You could really go wild with the usefulness of the rangefinder...especially if you rigged it with swappable units. You could have a camera (one of those tiny digital key chain cameras pointed out the front (when the RF is down) so you could take pics of an event...or a digital thermometer or the watch or a host of other useful small portable electronics.
Or a small mp3 player hooked up to speakers in the helmet?
I mean, it's a costume but the helmet is the most bad@$$ part so why not rig it up with all sorts of neat stuff?