While it’s common for people to collect things like books or art such as prints, there is someone, somewhere, collecting almost anything and everything one can imagine. My mother collected Hummel figurines and Delftware. My father has an extensive collection of old toy trains, along with tin toys, WWI and II militaria, and most recently, Gumby items!
More offbeat collections I know of range from empty perfume bottles to used Starbucks gift cards, both of which I have sold to collectors in Europe.
For horse lovers, a wealth of entertaining themes and items exist no matter what your breed or discipline. Racing fans search for programs, tote tickets from famous horses or races, track promos, as well as saddle cloths and actual halters worn by top horses.
One engaging category is paper ephemera, paper items used briefly then originally destined for the recycling bin. Advantages for collecting this category are items may not take up much space (think postage stamps or postcards), shipping is affordable, and a variety of pieces can be grouped into a single frame.
For horse lovers, George Ford Morris playing cards, farm brochures, stallion advertising cards, promo items given away at big horse shows advertising a show horse, breeding stallion, or farm are all fun things to gather and peruse. One collection I am working on myself is a group of sale catalogs, big, glossy promotional brochures and other things from the renowned Lasma Arabian farm. What are you collecting?