There are so many stories of turtles mistaking plastic bags for jellyfish, fish mistaking microplastics for plankton. Dolphins and turtles getting tangled and mangled inside plastic rope and rings. When wildlife eat plastic, they can become malnourished or starve to death, and their bodies absorb the hormones released from the plastic. If the wildlife happens to be food in our own food chain (i.e. fish), we absorb these hormones too (bioaccumulation). Liver damage and estrogenic effects (feminized males and decreased neonatal function) are some of the common negative effects.
2. Waste Management. Encourage your local waste management system to adopt best practices if they don't already. Ask your locally elected officials to support this. (Also, ask for a tour sometime of your waste management plant--it's fascinating and makes a great field trip!)
3. Design. Plastics manufacturers blame the user for littering or not recycling, but if the items weren't non-biodegradable, single-use, and/or "disposable," in the first place, these problems would not be issues.
Several of these images I've used with permission from 5Gyres.org. Thank you for your support, 5Gyres!