I’d go ‘shopping’ in the Alexandrian Library, Collected Works of Socrates and such…
I go back to about 1928 and give Hitler the $100 to continue his art career. I’d give him the backpack too. I’d try to mention how I was Jewish and I saw great potential in him as an artist.
Bravo.
The aging problem of collectibles (i.e. nobody will accept your mint-condition Action Comics #1 is real) pretty much rules those out. I figure your best bet is collecting something that just doesn’t exist any more, like Dodo DNA, kinescopes of now-lost TV shows or prints of now-lost films.
Such as the lost final reel of The Magnificent Andersons (1942).
I’m with davidm, go a few months into the future, surf to a website that aggregates winning lottery numbers and printout the results. Just for kicks you could add stock prices, sports outcomes, natural disasters and other events. Even paying a few cents a page at an internet cafe you should come in well under your $100 budget and even have enough for a drink and snack while you search.
If you’d rather go a few years in advance you could gather more information although the older (closer to your real time) information might be harder to gather.
Getting around the “aging” problem with collectibles: coins. Go back to the 1820s and pick up some $5 gold coins. Gold doesn’t “age”, so just say you found 'em in grandpa’s safe or something. Should get $5-10 million apiece, for 20 coins.
The killing-spree ending?
In three hours? 
RE: The comic books: couldn’t I just say my dad knew they’d be valuable when he was a kid and put them in a protective sheet? No one would really believe they were real? That’s a bummer.
All I know is that I’d bring a towel. I don’t want to do any traveling without a towel.
You will waste your entire three hours looking for a “comic book store.” 
Especially the T Rex pics, what with all the elaborate and colorful plumage and all.
Does this time travel thing involve going through one of them nekid scanners or getting my junk groped? If so, I’ll pass.
Be sure to make sure it’s free of smallpox.
You could just do this the other way - go back to the day before the last big lotto jackpot any buy a ticket.
Umm, afaik, a generic $5 US Gold Piece “Half Eagle” goes for around $500 or so. Mind you, that’s a nice return on your cash. $10000.
http://1920store.com/1920s/coins/gold-coins-coins/
Old comics will have aged in a certain way, due to the rather cheap paper- even if sealed. Of course, they’d have been sealed in a paper envelope anyway, no plastic.
Same thing with one of the first few printed for public distribution copies of the Declaration of Independence. You could get $5-10 million each.
Another problem with bringing back a stack of Action Comics # 1 (the first Superman) is that you will be flooding the market. The reason they’re so valuable is that there are so few of them. If suddenly you appear with a couple hundred mint condition copies, the value will plummet.
You’re looking at the 1920s. I was talking 1820s, something like the 1822 $5 coin cited here. I was originally thinking “pick up a stack of 1933 double eagles”, since there is precedent for a bunch of them suddenly turning up in uncirculated condition, but they weren’t something you could just walk into a store and buy, and unloading them would be kinda difficult.
A problem with a bunch of the “go take pictures” ideas I see is that you would basically be admitting that you were a time traveller. I imagine that would be more valuable than the photos. We never stipulated that the time travelling had to be secret, but it was kinda understood to us while talking about it.
I still don’t think we have a definitive answer, do we?
Yes, and apparently the US Treasury woudl think them stolen, and they’d have to be, since they were never circulated, so 1933 double eagles are right out per the OP.
In pretty much all case, any really rare coin would be very difficult for a time traveler to procure in just 3 hours. Pretty much, you’d have to accept more or less common coins in circulation at that time. So really rare coins are out, although certainly some of the 20 Half-eagles would have a nice collectors premium. Still, maybe that would bring us to $20-$50K.
I’d go back and get a Gutenberg Bible. If I could sell it, it would be for tens of millions. If I can’t sell it, hey, I’d still have a Gutenberg Bible.