It is the tradition in my family to buy my husband a Lego set to put together on Christmas morning. I saw this Lego particle accelerator by JK Brickworks, and thought it was perfect! (The very fun video for it on the Lego site is worth the click.)
I ordered all the parts for it, but I had to order the track from Germany, and it did not arrive by Christmas day (actually, it still hasn’t arrived). I gave my husband the rest of the set, so he could at least be entertained by it.
My husband has a 3D printer (that he built!), so he decided to design and build his own track. The cat was fascinated, and I think it ended up working really well!
Space&Aliens 
I didn’t know there was such a things as kits to build 3D printers. I built a couple of computers and even a robot, but that was a long time ago. I wonder if I could manage a printer? Hmm, food for thought.
My husband built his first one using old printer parts, among other things. He’s used it to build two more since then and is now working on a new design for number 4. He recommends looking at reprap.org and the printrbot.com Simple Metal. Self replicating machines are cool!