St. Marks NWR is one of our favorite hotspots to go. It's less than a 1/2 hour drive from our house and yields a ton of wildlife. I hate to sound so lazy, but since it's the south, I love to go somewhere that we can drive through. It's just too dang hot sometimes to walk and St. Marks has plenty to see from the road.
St. Marks has a variety of habitat types, which provides lots of opportunities to see lots of birds. The Visitor's Center has a nice pond out back. We've heard an alligator bellow out there, a couple of Barred Owls, and lots of migratory birds. Don't ever miss the Visitor's Center :)
Next the main road goes along through a wetland forest for several miles. This is a great place to drive slow and listen for woodpeckers, Brown-headed Nuthatches, and other forest birds. There are a few trails that come off the main road, we've never hiked those trails as it has been too hot, but they are hiked regularly.
Further on is a huge Bald Eagle nest. This is a great nest, as it is pretty far off and isn't accessible, which means people won't mess with it. There are also lots of tidal flats along the edge with pull-offs. The tidal flats are good places for a variety of shorebirds, herons, and egrets. We've even seen Red-breasted Mergansers swimming around there!
Anyways, lots more flats, wetlands, lakes, and then the road finishes out at the lighthouse and Gulf Coast. In front of the lighthouse, we have seen Seaside Sparrows and Black-bellied Plovers.
Overall, it's a fantastic refuge and definitely a place to visit if you are in the Florida Panhandle. It opens early and you can spend the whole day out there. Do remember that it is fairly rural in that area and there isn't much around for hotels or restaurants, except for nearby Tallahassee.
For more info, check out: https://www.fws.gov/refuge/st_marks/.