There's a specific outfit Korean girls wear to walk to the convenience store at 11 a.m. on Saturday and it is, quietly, an outfit. Sweats but the right sweats. Slides but only one specific kind. Bucket hat tilted slightly back. A bag big enough for a tumbler but not so big it looks like you're going somewhere. The point is to look like you just rolled out of bed, except that the rolling out of bed had three considered steps.
I used to hate this look. Then I moved to Seoul and I started doing it on weekends and I gave up. It is, to be honest, the most comfortable way to be perceived.
Get a matching set if you can, but the colors don't have to be perfect — slightly off-tone gray tops over slightly different gray bottoms reads more lived-in than exactly matching, which can tip into pajama territory.