This safe features a working 3 number combination lock. It is also constructed in such a way that you can't just take it apart. Make sure you determine the combination before you lock it! Otherwise you might have to deform/break some bricks in order to get into it.
I'm using the printed dish from the mini Millennium Falcon set (4488) on mine, but most people will probably just want to custom decorate a dish with proper numbers. There are are few other printed dishes that might be suitable, but none of them are very common.
I also built the frame in black for mine, and clad it with black tiles, but I've simplified it in the instructions for clarity.
You can change the combination by changing where the 'tabs' are located on the discs. Using the axles like I have done only really gives you a few options of where to put them, but you can use other parts to create the tabs as well.
You can watch the video to see exactly how the mechanism works.