Great project starting for kids young and old, the New Zealand Garden Bird Survey. Check out Landcare Research's website and use the keyword "bird survey". The survey runs from 27/6 through to 5/7 this year. There's a number of resources available from the website (the poster on the left is available there for example).
Whilst thinking of matters natural you might like to learn a bit more about Monarch butterflies. Apparently, these butterflies used to head for Butterfly Bay in Northland - where else you might ask! Numbers have fallen significantly and there's a lack if understanding of butterfly numbers and distribution. Follow the link to learn more and update the Monarch butterfly website with sightings. Again, a great activity to do with your kids.