Manatees are always relaxing, but do you know what works just as well? Jellyfish. The Newport Aquarium in Cincinnati seems to have changed, but when I was there a few years ago they had an amazing design. One large room was an otter exhibit. The plastic rocks and plexiglass walls created a serious echo, and the otters, being active and playful, had everyone screaming and talking loudly.
The next room was the Jellyfish Gallery. Dark, quiet, and with maroon velvet seats and plush walls it was a sharp, and welcome, contrast to the otter room. Walking through you got wired, then relaxed just as quickly.
Of course keeping your own jellyfish can be pretty stressful, but the jellyfish lamp is a decent substitute.