Have you considered taking him to obedience classes? It may just give you and him some more structure around what is expected of him. And I would really address the pulling on the leash. There are some simple "tricks" you can try which only take LOTS of patience. Just to do a Google and you can easily find some tips. If they don't work you can try the different collars that are available, but you do need to know how to use them. And if that doesn't work either I suggest you get some help from a professional trainer. I give my dog lots of off leash exercise to get rid of some of her energy but I agree with others that asking her to focus and practice self control on walks seems to wear her out even more. Heeling exercises, come and stay, etc.
For boredom busters you could try kongs indeed - my dog doesn't particularly like them, but lots of people swear by them. My dog's favourite is the treat ball. Only buy a good quality one or it will be chewed to bits. There are lots of other boredom buster toys available these days. But the bottle and frozen treats idea is much cheaper and more versatile of course.
With the growling at your niece, I would take him to places with other young kids if you can. Never allow them to tease him and try make it a really good experience. Maybe get some kids to throw the ball for him, give him treats, etc.