Back when I was making a futile attempt to hide my despair I actually came up with all the boxes and keys, and a couple of the riddles. The riddles are all rubbish, but I quite liked my idea of the plutonium chest with a uranium key. That's the kind of puzzle I want my legendary heroes tackling.
(If you know what's going on, then it's simple; make a copy of the key from a non-radioactive material. Preferably from about a mile away. The tricky part is if you don't recognise a plutonium key when you see one)
 In full supervillain mode I am now plotting a lock that is sub-critical when locked, but critical when open.