Nothing with keys. I had a master key to my school (we had stolen a master key and filed off the "DO NOT COPY" label on the back then copied it). Kept it under wraps for the two years I had it. I had passed it down to a junior who I was friends with.
Yesterday some kid got caught with one and ratted on everyone who had one (apparently, many copies had been made (even though the students we gave them to promised not to copy them) off of my original and my friend's original (which had been passed down as well). All in all, there were 12 people with keys that got snagged.
Now I'm banned from the school for life and possibly facing legal trouble because I was named as one of the originals. The only thing that might save me is how incompetent the news has made my school look in the past few years (Last year we had a case of Teacher-Student Sexual Relations, the year before that, our former principal had been stealing funds from the school in order to build and install water lines for a shower in his office), and this would just add another huge sting to our school.
So yeah. Avoid anything with master keys. Bad idea.