This is very timely for me! I started a new exercise class on Saturday and was very worried that my exercise capsule would be out of date! Last time I was a gym-goer I wore Athleta long leggings and tech tees or tanks. I was worried that leggings were totally out, but when I got there most people were still wearing either leggings or bike shorts. Most people were wearing tight/bare tanks on top, but I felt fine in my graphic tee. (It was mixed ages -- I might have been the oldest and there were people of all ages from 60s down to 20s.)
For walking in my neighborhood, I generally wear shorts and a tee if the weather is nice.