I can definitely do tax returns, and I just stuck a copy of "proof of rent paid" from 2010, and maybe a copy of our purchase agreement when we bought our house I've got like 10 copies of that floating around so I might as well do something with one of those! I've got a picture dated 08/2003 I'm sending, which is pretty hilarious paired with our most recent photos.
I wish I could remember what I sent the first time we did this!
It's a good thing you don't remember!
The requirements are much more stringent now and I think that people who have been through this in the past are actually at a disadvantage because it was so easy to get a spouse visa back then.
Other ideas, anything joint, so joint bank accounts, credit cards, insurance policies,-house, car, life, medical.
Wills. I even sent one of our joint AAA membership letters!