Chattanooga rewards visitors who leave the car alone for a day. You cannot say that about most cities this size in the South.
Start with the geography, because it explains the rest. The city sits in a bend of the Tennessee River with Lookout Mountain, Signal Mountain and Missionary Ridge rising directly out of it. Each has its own road, its own neighborhoods, and often its own weather. If someone tells you it is raining, ask where.
Downtown, the riverfront, the Bluff View arts district, the pedestrian bridge and the North Shore are all connected on foot, and walking that loop is the single best introduction to how the place is put together. A free electric shuttle runs the length of downtown, which solves parking for most of a visit.
The mountains are a separate day and require a car or a plan. Lookout Mountain holds the attractions most visitors have heard of; the trail systems on Signal Mountain and across the gorge at Prentice Cooper hold considerably fewer people.
A practical note on timing. The riverfront is the reason to come between April and October. Between November and March the city is still good, but the case for it shifts indoors, and a first-time visitor planning a January trip around the river is going to be disappointed by their own itinerary rather than by the city.
The finished guide will lay out a two-day and a four-day version, with the outdoor and indoor options kept separate so a trip can be built around whichever one the forecast allows.