the ring...

If it is a modern city, John attaches a machine to the box which would scan the holder's retina. If the scan result matches Mary's retina, it would open otherwise it wont.
And if the city is old-fashioned, he need not give her a ring, he might just propose her with flowers.

WOW....u r watching CID quite frequently these days...huh..???...impressive idea...but..u just think that if john called mary to his house ..... then why didn't he simply gave her the ring at that time only...he could have saved a lot of money..by not installing the system.......
still..again i appreciate...ur thinking...cool idea..but...u r thinking beyond john's BUDGET...

it's better that you solve it yourself..!!!!!

well...i have the key to this question...but still wanted everyone to workout with their brains to help out john... :p

then john would have to tell mary to break the box which contains the key....!!!!!!!:D

It is so simple!!!!!!1111

John,the idiot, goes to Mary's home and gives her the ring.

Mary would have loved that.....if John went to her house...but as I said that that the city is full of thieves so probably the thieves may loot him on the way....then...what will he do...???...so...better option was a strong box with a strong lock...I mean really strong...hard to break....
Your thinking was gud and simple......
But....try to solve it in the 'box and lock' way.....

yeah..well thats the ri8 answer...