Whether I'm heading to the post office or running to Whole Foods, I like to keep things light when I'm running errands. One of my go-to's are soft knit dresses, like this one from Lou & Grey (LOFT's Athleisure line). I just throw it on and I'm instantly dressed. I paired it with these tan sandals from Madewell and this crossbody bag from 3LilyPads (we recently did a giveaway with them).
A cross bodybag is always a must and it helps keep you handsfree, especially if you're doing groceries and don't like leaving your bag in the shopping cart. Overall, just keep things simple and comfortable.