What is your book? When I use speech bubbles, I like to use pictures of my students (sometimes we get those small sticky-backed photos after picture day). Then I think of a theme for one of my bulletin boards. One year I did Olympics. We studied all of the different sports. The kids chose their favorite sport. Then with their help I changed the bulletin board into a large scene of the Olympics. Kids drew and cut out pictures like big hockey rinks or the ski jump. Then the kids drew themselves doing their favorite sport. They used the photo for their head. Out of their mouths they had drawn speech bubbles. They used a small white piece of construction paper and wrote down what they were saying to someone else at the Olympics. They put these next to their person. Then the whole board ended up with speech bubbles all over it. Kids used quotation marks around the actual words they said. They got to take turns pointing to their person and said what was in their speech bubble.
So you could do the same with any theme. One year they were presidents, another they were astronaunts in space. They liked the picture part of their heads too. That way there was no question about who was who.