The Miami Museum of Science & Technology:
Don't waste your time or money, unless you have some very insistent kids tugging at your pant legs, wanting to see the pitiful selection of birds & tortoises out back.
While I was there, they were featuring a dinosaur exhibit ('Make them move!!!'), which consisted of a handful of sheetmetal dinosaurs that, with the pull of a lever, made two of them butt heads... that's all...
To make a full day out of only a little, though, the Vizcaya Mansion & Gardens are across the street - I didn't visit, but a bigshot businessman/developer in the early part of the century built a large European-style home, now a museum, and still one of the biggest houses (& tourist attractions) in the state of Florida.
These sites require a drive or transit ride from downtown Miami (20 minutes or so), so they're not very far.