Aquarium out of ols shoe box.. paint inside back blue.. hang pieces of green paper from inside top to be seaweed.. fishes out of sparkly paper, decorate inside bottom to look like the seabed.. crabs, snails, shells..
My DC made an elmo elephant out of the top of the plastic milk bottles the other day. the handle was the trunk etc....then covered it with big squares of brightly coloured paper! Especially good if you read the books! If not go for a short walk and collect various leaves etc and then just let them stick em on the paper! Great fun!