Over the years, I've progressively inherited 4 different cameras from my father, the last one is a DSLR, so the three others are pretty much unused now.
I can't bring myself to throw them away, but I'm not sure that there's anything more useful to be done with them.
For the curious, in order they are:
And the DSLR is a Pentax *ist DS
Well, if you can't part with them and you won't shoot film (you can share lenses between the *istD and the MZ-6, film isn't dead yet), then I guess you either box 'em up, put them on a shelf, or display them somewhere. However, one consideration for parting with them is there are often volunteer groups teaching poorer kids about photography that are grateful for any gear they can get, I've donated cameras and other equipment to such in the past.
Anyways, hopefully you're using the *istD, it still takes fine pictures.
Sell the cameras on your favorite auction site. These cameras may be useful for somebody else.
Hack one of them together with a slave flash, some ground glass, and a film positive, and project subversive messages onto popular tourist photography subjects.
The options seem to be:
See John Cavan's answer.
Personally, this is my favourite, because objects we think of as junk to be thrown away are really part of history. I put this into practice often. (My wife does not see things from quite such an historical perspective, however ;)
See John Cavan's answer.
This is probably the most constructive and generous idea so far. It's definitely the one to follow if you haven't got the space to keep them for posterity, you don't need to sell them for cash and you don't have a project to use them in.
See asalamon74's answer.
This is a variant of 2, but with a little bonus for you. :)
Evan Krall's answer provides an example. I'm sure there must be other projects like this, but I can't come up with them. This wouldn't be my choice, because I am a complete klutz, and totally incapable of projects like this! ;)
As @lindes suggests, get some film and keep on using them. Enjoy the deep colours, high resolution and all-round analogue awesomeness of the chemical camera!
If you can find film for them, (35 mm or whatever) shoot your photos, take the film to WalMart and have it processed and converted to digital, then selectively do whatever you desire with the results. You can get the digitals on disk, put them on computer and work with them there.
Create a personal museum. I've done this. Whoever is visiting will receive a tour of Antique flash paper through electronic strobes with Every flashbulb in the middle.
Many films and film holders glass plate to sheet To 116 and 828. I have every type Accept my jewel I sold on eBay, LeErerours daguerreotype camera. It was Priceless but I let it go for 6K.
In case any of these actually isn't serviceable:
Take them to bits, practice macro photography on the bits. Also keep all the hardware as spares - screws, washers, ball bearings etc used in cameras and lenses are often similar, and the actual bayonet mounts can be useful in improvising adapters.
Speaking of adapters: Keep the lenses, or check what they are worth before selling them all too cheap - these are often still useful to mirrorless users, and some fetch unexpectedly high prices. Oh, and some of the lenses might fit the DSLR - just read up on the so called "Ricoh Pin" issue before experimenting!
Oh, and: Careful with that old flash. Establish what voltage it puts on the hotshoe before even attempting to couple it with a modern camera.