The absolute best thing any adult can do is buy a GOOD set of allen wrenches. Good hand feel and solid construction combined with MUCH longer levers and it makes building furniture/whatever a joy.

Bonus points if you also buy a set of cheap hex head blades for your ratcheting driver of choice.

These days? The ikea/whatever allen keys get tossed in the recycling bin faster than I can remember if those are actually recyclable.