One of the things I collect are old (1870s-1930s) stereoview cards. I focus on railroad, Christmas, children, and bathing beach subjects. I have some nice train views from the early 1900s, and NYC Elevated views from the turn of the last century. The 3-D images make these photos more realistic, although they are not in color.
I also collect "linen" era postcards (1930-1950). For some reason, the buses interest me more than the trains (maybe because Streamlined Train cards are very common).