Green leaf veg can be great, spinach, kale etc Root Vegetables are good also, I would recommend feeding more leaf veg if possible.
The most important thing to remember is that the vegetables is broken down to enable your dog to use it better as dogs are not the best at breaking down plant cells (cellulose walls) ( a good example of this is when your dog eats grass and its comes out the same as it went in). The best way is too blend, pulp, mince or liquidise the veg raw to maintain it keeps all the nutrients, however boiling or steaming and then mashing can suffice as it will break down the