Columbia is great, and you HAVE to do O'Steens. Be aware that O'Steens does not take credit cards.
There is another restaurant, and for the life of me I cannot remember the name, but it is on Avenida Menendez, two story with a courtyard, catty-corner from the Bridge of Lions. If you find the Casa Monica hotel and/or Flagler College, walk toward the water. Went there one night when we didn't feel like bailing the car out of valet hell at the Casa Monica. Yummy fried shrimp and scallops, and not greasy. It's been ten years since then - hope they are still there.
Take the sightseeing trains. Corny, but they will point out all the cool stuff. Have also done the ghost tour - kind of neat, except it caused my spouse to start snapping digital photos at random, trying to catch an apparition.
Above all else - wear comfortable shoes, and drink plenty of water. Have fun!
