When I was a kid I was an avid sports fan, so collecting sports cards was not only a fun little hobby, it was also highly encouraged. I remember nearly every adult who saw me buying sports cards telling me to keep them in good condition because someday they would be worth a fortune. If only those grownups could have seen into the future and saw any and all value of 99.9% of sports cards fall below that of the cardstock they're printed on. To this day I still have boxes of (now vintage) sports cards that are in great or near mint condition that I couldn't sell for maybe more than a few bucks total.
Another investment I bothered to dabble in was comic books. While not a comic book reader myself I had heard much the same thing about comic books as I did about sports cards. For a very brief time I purchased them in large packs sold by Toys R Us to flesh out some semblance of a collection. I even bought those little bags to keep them all in and bought some cheap, dollar store backing to keep them from creasing in the bags. Like I said that was a very brief endeavor as I soon sold all my comic books off to a friend who originally inspired me to start collecting them in the first place. From what I remember I may have made $5 profit selling these to my friend, if that.
The one thing I did collect that no one ever really saw coming was video games. Now I'm not going to lie and say I saw the trend coming, because I didn't. I sold and traded away games I didn't play for games I would play more often, even if they were for a 10 year old system. So I just kept the games that appealed to me and most of them are still with me to this day, over 20 years later. As a kid I remember buying games from other kids who were more than happy to fork over NES games for super cheap because they thought they were junk and just taking up space in their closet. I, however, was overjoyed to be playing any video game whatsoever.
I think the view on video games at the time was that they were a fad and not something that would become the mainstay they are today. Video game consoles came and the old ones were pushed aside or thrown in the trash. Nobody knew, nobody cared. Moreover look at consoles that failed like the NES top loader that was sold in small quantities because the SNES was already out and not many people wanted to go backwards in technology. Or even the Virtual Boy, which failed altogether just based on itself. Nobody knew the failures of the past would be so highly desirable in the future.
I wish I could go back and talk to my younger self and tell myself that video games are going to be worth a lot of money, and they're an even more fun hobby than sports cards. Trade off those sports cards in the peak of their value and invest that money toward video games, because by the time you're in your late 30's your closet will have a lot of worthless sports cards because you sat too long on the egg and it never hatched.