Kaiapoi Island is no longer an island - but it is on the banks of the Waimakariri River and is sometimes called Waimakariri River Park. In summer it's a nice spot for a picnic and a swim or fishing - not to mention a bike ride, run or walk all year round.
The Kaiapoi Island Trail is a shared walker-biker track that makes for a loop of up to 19km along the banks of the river. It's flat, wide, easy, well surfaced with gravel and has no surprises apart from the possibility of running into a bike-eating poodle. It's popular with local runners, strollers, dog-walkers and families out for a ride.
The trail runs up and down the river bank and there are three popular access car parks. They're at Wrights Road, Taylors Road and Raven Quay. There is a toilet at the Wrights Road car park.
There is a brochure that you can download with description, map and so on. Just google Kaipoi Island to find it.