Looking to get into a Mini Computer, PDP or something like that

I just want something I can learn, I do love the blinkenlights and switches, something about actually flipping a switch instead of a keyboard I love. One of my dream machines is an Altair or an IMSAI 8080, but those are out of reach. I really dont know what I want to do on it yet, looks like there is so much to learn. My son is 9 and starting to learn unix and old nix. So something that could run an old version of unix would be really cool :). Something perhaps I could tie into our vintage telephone system as well. Im a total noob at mini's, but the thrill of the regular stuff has kind of waned, Ive put my amiga online, my c64, my IIGS, everythign I got goes online and games. and thats all I do with it. Most of it has sat in storage. Thanks for all the input guys