Training your legs is a very important part of any exercise routine. One session working your legs can burn energy all night while you are sleeping and recoverying.
Although its not ideal for your knees running is still one of the best forms of exercise, especially paired with swimming. When I used to Thai-box religiously everyone was after a six pack. They would do millions of sit-ups and my trainer would just send me out running. Everyone has the muslces that make a six pack, you just have to get rid of the sh*t thats around it!!
Also for grip and forearms couldn't agree more with a power ball and climbing. You will have forearms like Popeye (or Husky) in no time!! A would highly recommend a martial art though, never been as fit as when I used to Thai-box.