To relax, to not exert oneself.
let one's hair down
To relax and enjoy oneself.
kick up one's heels
To relax; to enjoy oneself; to do as one pleases.
let oneself go
To relax and enjoy oneself without restraint.
kick back
To relax; to laze.
bust one's chops
To exert oneself.
put one's mind to it
To apply oneself; to exert a directed effort.