I just detoured a 15 m rt to not-the-nearest grocery store, riding in my usuall summer errand attire: khaki shorts (neatly cut from long pants) and a rayon ss shirt, both courtesy of Goodwill. Rayon is wonderful hot weather material; which is why Hawaian shirts are often rayon. Flappy, which would be annoying on a longer ride, but tolerable for 20 miles (yes it was windy, as usual). Khakis go for under $7 (haven't bought pants at GW for a while, but that's safe; used to be $5 or $6) and shirts are under $5; and if you know what to look for you can get better quality than you can find at the local malls.
Other gear: Target cotton boxers, discount bin ankle-high cycling socks inside of 1980s Shimano SPD shoes (my faves because they fit so well), antique Rivendell cap, Tifoso photochromic shades, no gloves or helmet.