My snakes move the same outside as inside. If you would put your snake on the floor/bed/whatever inside without worry, the same can be done outside. Stay close and watch them.
FinFang (corn): Explores fearlessly, moves in slow circles mostly
Amel Boy (corn): Just want's to hide, I don't keep him out long
Moss (boa): Just sits in the grass and doesn't move
Thanagar (spotted python): explores a bit
Cardassia (boa): doesn't go outside because she's a super freaky ball of bitey stress
depends on the snake, your backyard set up, and your comfort level really.