Would be fun to get into train sets. Could have fancy shelves along all walls and have them go all over the house, have tunnels through walls and stuff. It'd totally do a polar express theme one or something.

FPV camera on a train set would be pretty fun too especially if you can fit it right inside the engine one. I wonder if they make some small enough that would fit. I've seen people manage to put a gopro style camera on one but it was kinda bulky.
Knowing me, I'd probably try to overclock it too. "Hmmm this says 9 volts max. What if I put 12?"