There is a nice walk up the Hamurana spring that you can see the trout in the water etc, took DH's 3yo cousin and he loved it
There is also a walk up the Ngongotaha Stream which according to my neighbour is lovely.
On weekends at the lake front there is usually a market, mini fire engine rides, and a couple of playgrounds that are neat for kids.
HTH - Tauranga or the Mount are only 40mins away so you could always spend a day at the beach if you are not keen on the lakes?