When my parents lived in northern Florida, we went to visit them for a week each year, and one of the places we always visited no matter what else was on the agenda was St. Augustine. It touts itself as America's oldest city (settled in 1565) and has charm for days!
Here's how my ideal day there would go:
First stop in the morning is St. Augustine Lighthouse:
Full disclosure: I'm afraid of heights a little. I'm also afraid of being at any height on stairs where I can see below me. So if you have similar fears, be like me and hang out on the lightkeeper's porch while your spouse and/or parents climb the lighthouse. There's also a nice museum in the basement (where it's always cool even on a hot Florida day) to keep you busy while they're up in the lighthouse.
Now we're going across the inlet and parking the car in the new central parking garage (way easier than finding a little hidden parking lot within the historic district, AND when you go back to your car it won't be a thousand degrees inside! #yourewelcome). Walk over to the Castillo de San Marcos, built to protect the Spanish settlement from pirates and the English army. You'll find barracks with some bunks and hay mattresses on display, weapon storage, and a prison (the creepiest part of the tour - there's a segregated room that I think was used for solitary confinement but you have to crawl through a tunnel to get in there. NOPE.)
The nice men in uniform put on a cannon demonstration every day to show off their artillery:
Once you've seen the fort, walk through the Old City Gates and take a stroll down St. George St. for some window shopping on the way to lunch.
Stop into Columbia Restaurant - it's right on St. George St. and yes it's a chain (I know, the horror), but the food is delicious and you'll feel like you stepped back in time and landed in a Cuban courtyard. I always get the Havana Club with a side of plantain chips and a cup of Cuban black bean soup. It will refresh you from your morning adventure and fuel you for the afternoon.
After lunch we continue down St. George St. for more shopping, stopping whenever we want for ice cream or water or whatever because it's probably 105 degrees by now. You'll find some junk shops just like in any high tourist area, but you'll also find some gems like the pottery shop that sells hand painted planters and dishes, the oldest school house, and just some damn fine scenery. It's several blocks of shopping, but in my opinion the best part is the 2 blocks closest to the city gates. Look how charming!
After all that shopping you're probably hungry again, so I'd recommend either Scarlet O'Hara's or the Bull & Crown for dinner. These have a much more casual atmosphere than lunch so have a beer or two with your dinner. You'll need to be a little loose for the next stop...
...which is a ghost tour! They have several ghost tour options but we prefer the trolley because you can cover more ground in the same amount of time. You get on and off at some of the more haunted spots and they have actors at each site to up the creep factor. I can't find our pictures but I know we stopped at the Old City jail, an apothecary, and a graveyard. There was another spot (can't remember which site it was) where we were gathered this room and a guy walked around telling a scary story and if you laughed he'd get up in your face and basically scare the crap out of you. I did not laugh. Along the way, the tour guide told stories about the city's rumored hauntings and we yelled 'BE DOOMED!' at anyone we passed who laughed or scoffed at tourists on a ghost tour. It was fun!
So that's my perfect day in St. Augustine. There are of course more sites to see, like the Fountain of Youth, the Alligator Farm, Fort Matanzas, Flagler College, San Sebastian Winery, but how long is this day?