And how much do we really know about the past? Can you remember the names of Edison’s partners? Maybe your time traveler already was one, and we already live in the changed world. You don’t need big-name changes, like Washington getting killed before the Revolution, or Colombia failing to remain united, to provide room to manoeuvre. There are plenty of holes and crannies to work in.
Note that there were some really serious financial panics toward the end of the 19th century. Anything in a bank or a brokerage could easily be lost.
Think outside of the box. Start a business digitizing pr0nography and sending it via the telegraph.
Become a songwriter. There are hundreds of extremely hummable melodies that we all know that hadn’t been written yet.
If your hypothetical allows you to steal others’ scientific discoveries, grow some bread mold and patent penicillin.
I’d imagine any of the german weapons guys would be a good bet in the early 30s.
IG Farben is the one that springs to mind first.
I would second betting on Presidential elections
Bet on the sinking of the titanic
You could short Cunard lines I guess as well
Bets on the 1936 Olympics would / should be a given
First, before you go back, research when and where ore and oil discoveries were made. Sites like Spindletop and Sutter’s Mill are obvious, but there ought to be plenty to pick from.
Once you’ve made a reasonable pot using the methods outlined above, make money by buying land above those sites and making sure to get mineral rights. Then, if you’ve socked enough away prior to this, you’ll be able to exploit those resources regardless of what havoc your time-traveling shenanigans have wrought on the timeline. Unless you went back a few geologic eras to the times those resources were being created and/or deposited, nothing you could possibly have done would change the fact of their existence.
In addition, it’s closer to being Wall Street-proof than a lot of the other ideas floated above. Car companies rise and fall (and a damn lot of them fell between 1900 and 1930) but coal and, later, oil always seem to hold a certain utilitarian value.
(For extra credit, do some research on modern extraction techniques and see if you can’t get any patents. You might even be able to turn otherwise unproductive finds into producing mines by application of the right future technologies.)
There are a couple of ways to look at this.
(a) You can’t change the past. It’s already happened, and so anything you plan to do will have to have already been done. So, before you go back in time, look yourself up, and see what you already did, and then do that. Of course it doesn’t really matter because you can’t change the past so you don’t really need to know what you already did because you’ll just do it anyway.
(b) Or, maybe you can change the past, because once you get there, you’re on a different timeline. Now the problem becomes ensuring that you don’t do something that will screw up your new timelime versus the things you remember on the previous timeline. For example, in our timeline you know Thomas Edison made his fortune, so you decide to capitalize on that in the new timeline. Perhaps at an investors’ party to which you’re invited, you offer Mr. Edison a shrimp appetizer, not knowing that he’s allergic to shrimp, and the anaphylactic shock kills him. Not only have you potentially ruined your investment opportunities with Edison, but possibly all future investment opportunities. E.g., now Tesla has no one to compete with, so he doesn’t electrify Niagara Falls, which causes a now unemployed electrical engineer to go work in Germany, where as senile, old man, he runs over Hitler before he comes to power. Now things are just as uncertain as if you’d stayed in your own time.
19th Century? Go back to 1980 when Andrew Lloyd Webber was begging people to invest in CATS, and put every penny you can into the show.
That was a once-in-a-lifetime opportunity for the 2,500 people who “kittied up.” They’ve got a 2500% return on their investment in five years!
Invest in a little-known paper mill named Kimberly-Clark, right before the Great War.
Take out life insurance policies on Titanic passengers.
Take out property insurance policies in San Francisco around the turn of the century.
Note to self: Avoid shrimp. Got it!
Oh, so you’d put your personal fortune over killing Hitler? 
They crashed twice. If you bought right after the crash, you had a double dip.
I’d simply bring some handy items with me and get patents for them. Lots of wealth and you’d probably be under everyone’s radar.
Investment, invention, gambling, etc., are all risky. It requires the state to enforce contracts, and you have no ties to give you strength to combat corruption. And you’ll have no way to collect a gambling windfall unless you found an honest bookie. Get to California before the gold rush and quietly get all you can. Once you have established some wealth, bankroll politicians. Then you can start investing.
We all know that bankrolling politicians is NEVER risky.
I would take with me as many Viagra tablets as I could carry. I’d give out just enough free samples to the right folks as “Dr. Sampiro’s Rejuvenative Elixir”, and charge what I wanted from impotent robber barons. (Cornelius Vanderbilt- a notorious skirt chaser and womanizer- was in his 80s and the richest man in the U.S. in the 1870s and though notoriously stingy would probably have paid dearly for a pick-me-up, and I’m guessing you could get a duchy if you got the right crowned head of Europe.)
With the Viagra money buy oilfields in Texas, beach property in Malibu and Santa Monica and Miami and the Gulf of Mexico, pay a lawyer to establish a trust, and go home to your own time. When come back to 19th century bring antibiotics to kids as a form of paying back for the money you’ve taken.
A good sci-fi/fantasy story could involve a sort of Triangle Trade: Take Viagra back to the 19th century, trade it for pounds and pounds of legally available and cheap cocaine, bring that to the 1980s, trade it for arms and gold, take the arms to the Jews on the eve of Krystallnacht (this being not for profit obviously) and then take the gold and buy land in 1870.
I wonder- if you took the atomic structure of Viagra with you would facilities have existed to manufacture it in the 1870s? Or would the Nazis be able to reverse engineer 1980s automatic weapons?
Or do what Sam Brannan did upon the discovery of gold in California. Hoard up all the equipment and supplies you can just before the gold rush, then start a business selling shovels and other equipment and supplies to miners when they arrive.
Maybe Sam Brannan was a time traveler. :dubious:
He also had a lot of Mormon money with him that, IIRC, he never repaid after he apostasized in spite of threats of lawsuit and eternal damnation and a lot of things in between from Brigham Young. I read a bio of him once and I remember his father was almost 80 when Sam was born. Brannan was also one of the first men to be wiped out by a divorce due to his “habitual adultery”; Mormon prophets used his late in life penury (after having been extremely rich) as evidence of “woe unto those who apostasize” morality tale.
Certainly not when you know who’s going to win the election;)
Find an obscure Dutch artist named Vincent VanGogh - buy everything he paints.