We travel to Blenheim frequently and have lunched at most of the vineyard restaurants. There are many good ones but after lunching here this week, we think Wairau River is the best (other good ones? Allan Scott, Hunters and Highfield Estate).
The menu was extensive and 'proper food' rather than antipasto platters which is what many vineyard restaurants offer. The staff were friendly and seemed to enjoy their job. You can eat indoors or outside under shade umbrellas, or even eat reclining in bean bags!
It's a great place to take children as there is lots of room to run around while keeping them in sight.
Choose a sunny day and head here for a long, relaxed lunch (in my home town).