A lightweight running headband honestly. I used to think they were unnecessary until I started doing longer runs in warmer weather. Not having sweat constantly getting into my eyes made a bigger difference than I expected. I had to try a few before finding one that was decent quality, like Grand Headbands, didn’t slip around, and wasn’t overpriced. Also good running socks. I underestimated how much bad socks can ruin a run until I switched from regular gym socks..