Most resistance training equipment can be used outdoors. But some still require a solid and stable ground (like Olympic Barbells). Advantages of kettlebells over other resistance training equipement is that it requires very minimal maintenance cost. It can be used at the beach and would have minimal damage (a very thin rust) if not wiped clean after getting wet with sea water. You can practice juggling with it too. Here is a sample workout when you want to have fun at the beach.
valstrengthtraining 