If you want to see glowworms, you might have heard of the famous glowworm caves in Waitomo, where you have to pay an expensive entry/tour fee. Although these are a nice visit anyway, there is also a free cave in Northland where you can see many glowworms too: Waipu Cave. And the good thing is: you can go here all by yourself! Bring a torch and some good shoes (it is muddy in the cave) and explore them as long as you want. There is even a free campsite in front of it.