An Investment Mattress - You're not old, but your poor, technology-wrought spine has seen better days. If you invest in nothing else on this list, a good mattress is the one item we do actually think is critical to own by your 30s. We're big fans of the Casper brand, but if the price is giving you sticker shock, shop around for other direct-to-consumer sites for lower prices. (As always, use Honey to look for discounts.) Extra adulting points for a headboard, too.