Pictures of the house before, during, and after building. A self photo, any info. related to the neighborhood, copy of the house plans, a dollar bill and some change, anything that someone who owns the house 100 years or more down the road will be fascinated with.
I have an old house built in 1897, and have found a dated signature from the original owner of the house on a corner of one of the floorboards, and have found a 1908 newspaper, and a 1927 newspaper stuck in crevices, and am still looking for the "Big Find" of some kind of heirloom.
Anything to do with the house, and the original owner will be prized and cherished by the person that finds it many years from now...