You will arrive in Matiatia from the ferry. From there you can jump on a bus up to Oneroa, which is the main village on the island. (will take around 5 mins - you can walk it in about 20.) Buses meet each ferry as it comes in, and travel around the island regularly, so it's quite easy.

There are lots of shops, cafes, galleries in Oneroa to have a look a around. From there, you can also walk down the hill to Oneroa beach which will be great for a picnic. (The path is quite steep though, so an easier access point would be down the street on the left once you have walked through the village). There is a swingset, and toilets but no real playground at Oneroa.

Alternatively, you could catch the bus all the way to Onetangi beach on the other sideof the island. It takes around 30mins. The beach is bigger and beautiful and there are a couple of cafes there.

Most of the vineyards are not within an easy walk of Oneroa. Mudbrick and Cable Bay are a bit of a hike along a winding, exposed road - so wouldn't recommend trying to get up there. I think you could probably jump off the bus and pop in to Stonyridge if you were on your way to Onetangi though.

If you are going to arrive on Saturday morning you should check out the markets at Ostend (also on the bus route), but they finish by midday.

HTH!