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.