Most people tend to frequent Soutland after moving on from Catlins. One of the first stops is usually Bluff, which is the 2nd most southery point in NZ that most people frequent (Slope point is the most southern on the south island and is a decent walk in the catlins to get to it). There is a good cafe nearby to enjoy the views from. You can also see Stewart Island from here on a good day.
Invercargill doesn’t have too much to offer but is the main hub in the Southland area. Its a good place to stop off and resupply.
After Invercargill, Riverton is the next biggest place you will find. It is a nice seaside town, and it has some very interesting volcanic remains; Riverton Rocks. These rocks are very odd in that they are blue/green/purple/brown in colour. And they look like balls of popcorn mashed together.
Colac Bay is a bit of an undiscovered jewel. A sheltered quiet cove that offers a nice retreat on your way to Te Anau. Would recommend hiring a holiday home here if you are passing by. The coastal area just before and after here, has many a beach with interesting differences from the rocks and shells that you find washed up. For example gemstone bay, just 15 mins drive away, actually has precious stones that wash up on the beach every so often.
The best part of Southland is actually the Fiordland National park area. This has its own separate write up.