I'll use this blog post to offer suggestions about the possibilities of starting a new postcard collection or items to an old one. Home Schooling families could use them to learn geography or 20th Century History.
Of course, you could collect your hometown. Some towns have thousands to choose from such as Post Offices, Libraries, Schools, Main Streets, Laundromats, Bakeries, Churches, Colleges, Drugstores, etc.
Series postcards are collected by many: Numbered Curteich or other Large Letter Linens, the Artist Ken Haag State Flowers & Birds from the 1960s (All 50 States)
Jewish postcards, (especially by Artist Morris Katz) could be a worthwhile pursuit.Look for caricatures on comic postcards. Synagogues cards can be expensive, but are well worth the time to find them.
There are thousands of chain stores that you could collect: Isaly's Ice Cream, Walgreens, McAn Shoes, Sears, Woolworth's, Penney's, Rexall, A&P Grocery Stores, etc.
Transportation postcards are sometimes highly sought after: Trains & Depots, Trolleys, Double-Decker Buses, Steamboats, Riverboats & Ships, Bus Stations, Airplanes & Airports. Motorcycles are harder to find, but could be quite enjoyable. There are collectors of specific makes or kinds of Automobiles: Woodies, Hupmobiles, The Edsel, Corvettes, Corvairs, Volkswagen Bugs (Beetles) and many 1950s cars.
I know one collector who is trying to obtain a postcard of every courthouse in the United States, including ones that have been demolished. Others collect Department Stores,
You can collect a favorite publisher such as Detroit Publishing (Detroit Photographic), TUCK, PFB, Whitney-Made, or Rotograph
There are millions of greetings postcards available for collecting: Christmas (Santa, Angels, candle-lit trees), Easter (you could focus on fantasy or dressed animals), Thanksgiving, 4th of July (or other patriotic postcards), St Patrick's Day, Mother's Day (Rare!), or Memorial-Armistice-Veteran's Day-Decoration Day.