on one's toes
• ready for any eventuality; alert.
• "he carries out random spot checks to keep everyone on their toes"
Similar:
alert,
quick-witted,
sharp,
bright,
quick,
keen,
perceptive,
wide awake,
responsive,
agile,
acute,
astute,
on the ball,
quick off the mark,
quick on the uptake,
all there,
with it,
bright-eyed and bushy-tailed,
whip-smart,
