Do something that you really love, something that you are passionate, do an activity that is not giving you any resistance or something that will not give you any form of regrets. Most people do something because it is what most people do or it is something that looks very cool when you do it. A lot of people do MMA, listen to k-pop, triathlons or anything that is in demand in the present because they think it will make them look cool, strong or they will get a lot of attention by doing it.
I am not saying that those hobbies/interests are bad, I am only saying that just because it is cool to others you will do it too. Don't try to force yourself into something that you really don't love, just because celebrities wear trendy shirts doesn't mean you have to wear it too. Being cool is different from trying to be cool. You are cool already if you are very true to yourself. So what if people tells you that you are not cool or you are out of fashion or what you are doing is a boring stuff. Just tell them to mind their own business and stop faking coolness.
The real meaning of being cool is doing what makes you feel good, choosing what your heart says and ignoring the things/stuffs/people that is out of your taste. Always taste what your taste tells and never get influenced by people around you. If you feel like wearing a 70's polo shirt with lots of flowers then do it. If you feel like listening to 1950's music then do it. If you want to do an old fashioned hobby such as weaving, building puzzles with million pieces, boogie, disco then do it. There are no rules about being cool, we also don't have any generation to follow, just be true to yourself and never try to always fit in.