Microplastics could be used as epoxy filler to make industrial products that don't end up in ocean, such as picnic benches, decks, picture frames. There are social behaviors that could use some modification to prevent them from getting to ocean, and much more could be done to pressure plastic/petroleum companies to be responsible in their cradle to cradle manufacturing or production.
I almost forgot to post here, one amazing thing you can do with microplastics is to make a Sea Globe -- read about @maxliboiron's beautiful (if troubling!) means of displaying microplastics in a remix of the "snow globe:"
This series of sea globes are genuine New York City souvenirs. The plastics came from the Hudson River in south Brooklyn, and the rocks are made of bituminous coal from in a landfill that closed in the 1930s at Deadhorse Bay, which now resides underwater at high tide, also in south Brooklyn. The snails are from a well-known taxidermy shop in SoHo. Overall, the sea globes are accurate representations of the waterfront environment in New York City today.
This work was created specifically for Gyre: The Plastic Ocean at the Anchorage Museum in Alaska, February 7- September 6, 2014.