Hi louton,
Well - the great thing is that all these issues are connected, and if you manage to do something good for one (packaging for example), it is good for the others (climate change and clean oceans) too!
One of the rules of campaigning is to do things that you are passionate about and that you can change. As young people - we have massive influence on our families and schools, so how about doing a check in every home in your street if they have good insulation (which means we don't have leaky homes, keeps people warm and comfortable without extra carbon and bills - mum and dad will love that!).
Or something about cycling access to school? Or recycling, or understanding climate change, or collecting signatures telling your MP that they should do more on the environment, or a film night with movies that are about climate change?
I hope that gives you some ideas!