Something vastly different from 1990 to today is photography. Remember when going on vacation or to some event, you were limited to a few dozen photos, so you had to police yourself on taking them? And you had to wait until they had been printed to see if you got your shot or not? If you didn’t have a “One Hour” type place nearby, you would have to send them off and wait days? And showing people photos required phycically showing people photos? And a camera was something extra you would have to have and carry around instead of something always in your pocket?
Oh! And you had to anticipate the lighting conditions ahead of time and buy film with the appropriate ISO and color temperature.
Having 2020 Marty mess up taking pictures because he’s used to taking a dozen shots, picking the best one, then retouching it could form a comedy bit or a plot point (like he needs a picture to prove something). Similarly 2020 Marty is going to expect anyone to just ‘take a pic’ or ‘take a video’ and not need drag along a camera, flash bulbs, and film or a camcorder and tapes. And to be able to show his pics on a tv screen or computer without much trouble, instead of either holding a bunch of prints or getting a slideshow made and dragging along a projector and screen for it.
My original thought was 'who actually looks at the tiny ‘Series 1985’ note on a bill - I’ve never looked at it outside of times like this thread, and I’ve never seen anywhere that has cashiers look for the date on bills. You might catch it on coins, but I really don’t think that anyone would look for that tiny date on a bill And I expected that the 2020 Marty would be unlikely to be able to pass off anything but $1 bills in 1990 because the design of higher bills have all gone through major changes. However, when I looked up the history of the dollar bill, there were major changes to all denominations in the 1960s - notably the ‘Federal Reserve Note’ and ‘In God We Trust’ were added, the border was changed, and the design and color of the treasury seal was changed. So 1985 Marty actually probably would get caught for ‘fake’ money, not because of the dates on dollar bills (which no one looks at in the normal course of business) but because the bills would look significantly different at a glance.
I was stationed in Germany 1991 to 1993, and we had plentiful Evian in 1.5 liter bottles, because the German water supply on base was hard and just not really all that good to drink. We joked that it was “‘naive’ backwards” even though, but bought it anyway, because in Germany, it was just cheap, bottled water like Nestle is here.
Also, Capri Sun didn’t come in stupid, little envelopes, but was sold in large bottles, too.
I have to agree with those that say it can’t be all tech-based and has to be cultural. Smoking was a great choice to pick. No need to retread the same jokes - him ordering a drink, etc. Maybe people driving drunk, but not sure there’d be an occasion for him to see it (maybe the dance).
And I disagree with all those who are making jokes that either make him look like moron or don’t reflect ordinary 2020 living - most middle or working-class American teens know how to use a light-switch. And dial-thermostats would be probably as understandable as rotary phones in 1985.
“Gweneth Paltrow is selling what???!!!”
Maybe there could be a bit where Doc makes some snide remark about how only bums, hookers and crackheads live in some rundown tenement in downtown Hill Valley, to which Marty says something like, “Really? In 2020 studio apartments there go for two grand a month.” Which prompts Doc to say “Great Scott.”
Well, I’ve been to Evian. Nice casino, spa, yachting, hiking, etc. And Evian water just CONTINUOUSLY POURS OUT OF PUBLIC FOUNTAINS! For free!! And comes out of everybody’s tap at home!!! 
Don’t know your age (I have a guess), but here goes:
How does one get to talk to an archaeologist in 1990? Well, been there, done that. Excavations not far from my home were in progress from the mid-70s into the early 90’s. Our local library (again) had a small exhibition about the finds, which I found fascinating. Provided was a map that told me exactly where to go. Walking there, I found a team of archaeologists at work and started asking questions. Had I not visited the library all summer, our local newspaper ran articles on the excavations, so there was no way I was going to miss this, and no way I did not get into contact with professional archaeologists.
How does one get to know other niche hobbyists in 1990? Been there etc. Reading the newspaper, I saw an ad for an archery demonstration, a busride away from my home. I went there, and started asking questions. In the couple of hours there my views on what kind of archery I liked (very niche) and how would I proceed from there solidified. Then - get this - I exchanged addresses with the guy who had the most interesting archery gear and stories, and we would do correspondence for a year while I was learning the ropes and more. Then I would attend my first archery competition, invited by my (then) only contact.
Not everyone has a home recording studio even today, and that is no biggie. Who needs to record and mix songs before they are in a band? Back in 1990 I was learning the bass. To get into auditions, I would record myself on a cassette, using a cassette player everyone had, and (gasp) mail copies of the tape to bands I was interested in to join. Then I would join bands and eventually we would book studio time, nothing to it. Any 2020 teenager could do it.
I spent my childhood in public libraries. Trust me, there were new, non-popular novels on the shelves all the time. I would learn about oppressed minorities, drug culture, kinky sex etc. from them. Where does this straw man of “a few bestsellers by white males in libraries” in the fucking 1990s come from?
It is laughable to think, and simply not true, that without the internet, a 17-year-old would miss out on these things.
Yeah, I shoulda heeded the pedantry here, and use “Blockbuster-equivalent” in my post. Attacking this reeks of clutching at straws, to be honest.
“I’m from the future! Stock up on toilet paper! Start social distancing!”
“How far in the future?”
“…uh, March.”
Well, we now have it.