You mentioned a bunch of physical artefacts that can sit around for years to be found later rather than being consumed at the time, but kids today still have experiences even if they aren't manifested that way. For example they could be nostalgic about:
-Fortnite
-Food / drink
-Unrestricted internet
-Possibly more privacy e.g. maybe in 30 years time everyone will be under constant gps surveillance
-Being a '1337 h4x0r' and feeling special for getting hold of music/games/films etc via nefarious means whereas in the future maybe most people will be able to legally stream it all (timing might be off on this one, I guess most people in that bucket are already adults, and we're fast approaching that point already)
-Writing things out by hand rather than digitally
-Maybe something Brexit related
-Potentially something we currently take for granted that has become much scarcer due to political change, wars etc (e.g. cheap transport, food, water, shelter). Difficult to envisage but suppose we had a world where things we use daily and waste without thinking about it cost 1000x as much in real times. Where say it cost the equivalent of £1000 today to have a shower, or to travel 10 miles by road/rail etc. I think you'd look back and go "wow, I remember when I could get up in the morning and just turn on the shower in the summer, good times" or "I used to travel around the country experiencing different things, now I never leave the county".
On the more physical side:
-The way people are turning against plastic, who knows, maybe in the future people will be carefully fondling plastic toys in their parent's loft, meanwhile their own children don't have any because they've been banned.
-Maybe physical money will be on the way out? Imagine a world where everything has been cashless for 10 years and you then stumble on a £1 coin down the back of the sofa....
-Clothes, I'm not up with the trends but presumably there will have been some stuff around in recent years that will go out of fashion and kids look back on fondly, just like we look back fondly on shellsuits...

-Theme parks, playgrounds and whatnot before they all get shut down under H&S regs