The historic downtown areas of both cities are full of photographic opportunities. In Charleston, the area known as The Battery and Meeting Street are especially scenic. I'm less familiar with Savannah as far as specific locations, but it is very similar to Charleston in terms of architecture. Both cities are very walkable, and I would recommend locating the visitor centers - they are well marked - and getting some advice directly from the locals.
I can't be as much help for wildlife, except to say that any of the coastal areas should be good for birds. Folly Beach or Sullivan's Island near Charleston, Pinkney Island Wildlife Refuge near Hilton Head Island or Tybee Island near Savannah would be good destinations. You should find plenty of flowers everywhere this time of the year.