Walking, running, standing, or sitting barefoot, will ground you.
Outside: grass, soil, gravel, stone, sand, brick, concrete.
Inside: unpainted or unsealed concrete or tile floor built directly on the Earth (best to test the floor for conductivity).
The following surfaces will not ground you.
Outside: asphalt, vinyl, wood (such as wooden decks).
Inside: wooden floors, painted/sealed concrete or tiled floors, carpets.