Let’s say you had one million dollars, to spend on something to collect. Given this what would you collect?
Coins? Door knobs? Umbrella stands? Paintings? Dolls? Ivory figures?
At first a million sounds like a lot, till you realize how much things can go for if they are antiques, and I assume you’d want a decent collection.
I think I would collect model trains. I always wanted to do this, but I’ve never had the money and certainly not the space.
Hundred dollar bills! 
Assuming it has to be something inherently collectible, probably vintage cars. I really like old cars, and they’re something just about everyone will ooh and aah over when I take one out for a drive.
Cars of the 30’s , 40’s, 50’s
Stock certificates
Experiences.
(And, maybe, pigmys and monkey butlers!)
Probably comic books.
Fabergé eggs!!!
Rare books. I already have a book collection, some of which consists of relatively rare and interesting first and limited editions, but I drool over some of the real rarities in my field of interest.
Things you could kill somebody with.
That is, things not designed to kill , but that are easily re-purposed for the job.
This. Or gold, diamonds, guns, etc.
To invest in or for personal amusement? If the first, I’d take the advice of art experts and start a collection. If the second, first editions of the great classic cookbooks.
Music on CDs or guitars.
Assuming I had a place to put it, baseball and bowling memorabilia.
That would not work well in my present living situation, however. Can I at least spend some of the money on shelving? 
Late 60s and early 70s American muscle cars.
Another bibliophile here. although I think I could easily spend that much on the library facility to keep them in . . .
Supercars, I know, I could get only 2 or 3, maybe 4 if I don’t go overboard.
Fine watches.
Art: paintings and/or sculpture.
Two chicks at once.