The Waitomo Glowworm Caves are one of New Zealand's great natural spectacles and easily the most famous day trip from Hamilton, about 80 km to the south-west. The main cave tour takes you through limestone chambers and culminates with a boat ride through the glowworm grotto, where the ceiling glows blue-green with thousands of bioluminescent larvae. It is as good as it sounds.
The Waitomo complex has multiple cave systems with different tour options: the standard glowworm tour, a more adventurous black-water rafting experience (floating through underground rivers on rubber rings), and abseiling trips into the Aranui and Ruakuri caves. The area makes for a full day — arrive for an early tour, then do a second activity, and be back in Hamilton by early evening. Book all activities in advance, particularly in summer.