Vintage non sports cards

[Musashi]

Depends on what your idea of a fun collecting challenge is. For example, "rarest" doesn't always equal "most collectable". With that in mind, some things that could be fun:

G-Men and Heroes of the Law: The set itself isn't rare, but there are some short prints that make completing the set a challenge. You can also go down the variations rabbit hole if you prefer games set to "hard"

Goudey Indian White Backgrounds are a nice challenge

T59's Flags of All Nations master set (every possible front/back combo) if you're looking for a challenge to pass along to the next generation

If you want rare - uncut T175 Heroes of the Spanish American War. Issued as box panels with Sweet Caporal cigarettes. The cut singles aren't something you see every day, I've seen an uncut package only once (caveat: non sport is not my focus, YMMV) or T11 Tales of the Turkish Trophies are tough.