Sally and the Some-Thing by George O'Connor
When Sally goes fishing she meets a strange sea creature and decides to make friends with the Some-Thing. They try all her favorite things--biking, coloring, drinking tea--but it's a disaster. Then they try all the Some-Thing's favorite activities like watching a snail race and logrolling but Sally doesn't have any fun. At last they make up new activities that both of them like to do. The pictures in this book are really fun, with the Some-Thing looking like a mix between a giant frog with a dorsal fin.