If you happen to be in Coral Gables in Dec or Jan, there's a spot where several manatee often congregate. On sunny days, it's fun to stop and watch them for a bit. I would not drive from the Miami Beach to see this - the manatees are unreliably present. They tend to be here more when the days are cool.
The "secret spot" is on Riviera Drive opposite the Coral Gables High School track. It's about 2 blocks south of Bird Road.