Mensa Alarm Clock: Only Geniuses Need Apply
Seriously, it's similar to that old Simon game. You have to remember and enter a sequence of colored lights to shut the darn thing off. However, if you get it wrong or are too slow, it'll sound the alarm again --- this time louder --- and louder --- and louder.
Of course, if you look carefully, there's a "Wimp Out" button. Press that, and you'll get a bonus of 30 seconds of an extra loud alarm as punishment.
Now, it doesn't seem to be available on the U.K. Mensa site, though they do have other gadgets for sale. Despite that, I assume that with www.mensa.org.uk emblazoned on the clock, it was indeed developed in concert with Mensa.
It doesn't seem to be available in any U.S. stores, but it's on eBay and at Amazon's U.K. site.
Strangely, the U.S. Mensa site is much more boring than the U.K. site; the only gifts it has for the holidays is a gift test set, no gadgets as on the U.K. site.