Placid. Also tranquil and recollected.
From Merriam-Webster online:
: not easily upset or excited
: not moving much : calm and steady
: serenely free of interruption or disturbance
collected, composed, cool, coolheaded, equal, level, limpid, peaceful, calm, possessed, recollected, sedate, self-composed, self-possessed, serene, smooth, together, tranquil, undisturbed, unperturbed, unruffled, unshaken, untroubled, unworried
agitated, discomposed, disturbed, flustered, perturbed, unglued, unhinged, unstrung, upset