Buy pet food in large bags or from refill stores, choose rope, wool and natural rubber toys, and for dog waste use the thinnest bags that do the job, or a home dog-waste composter where it is allowed.
Two poo bags a day at about 5 g each is roughly 3.6 kg a year per dog; halving that with a single larger bag per walk or a composter saves around 1.8 kg. Compostable bags only help if they go to composting: University College London (UCL)'s Big Compost Experiment found most compostable items failed in home heaps, and a compostable bag in landfill behaves like any other.
The catch: dog waste must not go in food-waste collections; compostable poo bags in a general bin are no better than plastic ones.