The Catlins area is a bit confusing as technically be partly belongs to Southland (the forest area) and also belongs to Otago (coastal). So it has been given a section of its own to help explain
The first place that you will get to is Kaka Point, which has a very lovely beach and the waterfront cafe/bar has some excellent food choices, so is a good place to lunch. From here is the only access to the very pretty and geologically interesting (uplift events and massive sea erosion) Nugget Point. There is a lighthouse here that is quite pretty but it is the view onto the “nuggets” of rock that are interesting. The view here on a clear day is amazing and you see for miles. Also keep an eye out for seals and larger seagulls.
From Kaka Point and heading south, you will pass by many other interesting places (there are maps of the “to dos” everywhere) such as many different falls. In particular the MacLeans Fall is amazing. Two big drops, logs strewn about, with tannin coloured water is a must see and explore. You can actually explore the “landing” between the two drops but be very aware of your surroundings. A fall is possible if you are not being careful.
Something that personally would not advise seeing is Jacks Blowhole. Unless there is a storm brewing. Went here at exactly the right time only to be very disappointed to just see a hole in the ground onto the ocean. Can see that on the west coast and its much better.
From MacLeans Falls, is a short drive to a historical logging site that has remnants left to play with (such as an old logging tractor and metal items). There are boards around to help guide you around this area.
Cathedral Caves are pretty awesome but the timing of seeing these is very strict and there is a $3-5 entrance fee. It is worth the money and time if you have it to spare. Unlike Cathedral cOve in the Coromandel, there is a lovely seaweed smell as you enter and if you are lucky you may even see some interesting cave insects or crabs. Just opposite the entrance almost is the whistling frog cafe, which is another yummy place to stop off at.
All throughout the area are awesome little villages/houses that make for great places to stay at and most have B&B type places available for quite cheap. Definitely try to enjoy these rather than the hostels that are focused on certain areas, as its not a “real” experience.
One of the most popular things in the Catlins is the Curio Bay. This place is rather special internationally as its one of the few places you can observe a fossiled prehistoric forest, and its right by the ocean. The downside is that it is slowly eroding away. In addition to that there is also a chance to see penguins if you time it right. Yellow eyed pengiuns are just one of the many species that you can see in many places throughout NZ.
There are other more "difficult" to get to places such as Slope Point (the south islands most southern point - not bluff as is commonly believed) or the Waipapa lighthouse. Excellent detours off the "main" route if you have the time.