Ngilgi Caves
the first 'g' is silent are just out of town on the dunsborough road. the caves have a long history of being show caves, attracting tourists from perth for over 100 years. the tours are semi-guided, taking you down in the caves, and giving you as much time as you like to wonder around and see the formations. you can just lie on the ground and look up at the formations from the roof of the cavern for as long as you are inclined. the kids will like the crawl tunnel. a joint ticket is available with the cape naturaliste lighthouse.
Snorkelling
Yallingup has the southernmost occurrence of tropical fish in western australia, and there is a protected reef just to the south of the main surfing beach. take your snorkel and have a look around.
Swimming
Or surfing the beach is sandy, with a reef break further out.