Thanks for the tips everyone. I'm back. Quick results.
St. Augustine: fun, people were nice, old town has lots to do. Beaches across the intercoastal were nice, but car access sucks. This may be the norm in Florida - great foot access, terrible parking access. Traffic was pretty ridiculous. Quite touristy, and this is off season, so it must be a madhouse at peak.
Amelia Island: lower half is gorgeous and mostly unspoiled, but houses are all in gated communities. Fernandina Beach is a fun downtown, found several breweries. People were very nice. Main beach was accessible and very nice. Astonishing how little traffic there was. Can get anywhere in a few minutes. Old neighborhoods of cute houses and lots of newer communities too. Just a very relaxing location. Yulee across the bridge is a modern suburb with everything you need.
St. Simons Island: More remote. Pretty and lush. Loads of live oaks. Feels more like a town that happens to be an island. Lots of homes of all types. Nicer beach than expected, some parking, good foot access. Downtown is ok, mostly a beachy vibe, not at all fancy for a place with lots of rich people. Only one public golf course. People are really active, out walking and biking and there are paths and trails everywhere. Lots of golf cart use as transportation.
Jacksonville: Nice airport. Downtown seems ok. Sports complex is modern and on the water, but the put it in what was probably a rough part of town. Went through "historic" Springfield neighborhood, which used to be "the hood". Seems a gentrification is underway. Amazed by the lack of traffic. Boston traffic at 11:10 PM on a Sunday was much worse than JAX at rush hour on Thursday.
Unexpected: the two Islands south of Amelia, Little Talbot and Big Talbot, comprise a big part of the Timucuan Ecological and Historical Preserve. Holy smokes the views of the water from there look like the Caribbean. Simply gorgeous.