If I want an old camera with light leaks I can always use one of my old Kodak Brownie Boxes. I picked one up on ebay for a few bucks and it is great. I also use an old Brownie Hawkeye bakelite camera. It is made for 620 film, but I can use a 120 roll as long as the take up spool is a 620 spool. Really cool old camera. Mine was like new in the original box on ebay for $10.
If you don't want to stand behind our troops, please feel free to stand in front of them.
While I've never used a Diana I got my start into MF with a Holga (I have a couple of shots from those days in my apug gallery) and am very glad I got to use something like that for a while. It really does make you think differently about photography and they're a lot of fun to modify and tweak. They're also great cameras for parties; there were a few times when I would bring one out to some big, outdoor get-together and just pass the thing around (with some simple instructions taped to the back...with a hole for the window of course.) If you can find it the next day you'll be certain to have some interesting snaps to share.
Check out some of the member images submitted in galleries here on APUG for a view of what Holgas, etc. can do.
One thing I've always been impressed by is something so simple and stripped down can nonetheless be really, really hard to truly master. An image submitted recently by jbridges, as well as some Holga images lithed and toned by fellow Ape Hugger thebanana are nothing short of sublime. I'd have to work pretty damn hard to even come close to that sort of thing.