We do no formal lessons in summer, but we do have a theme of the week. Keep the themes simple and let the kids help determine what they are so they will enjoy it even more. Sometimes our themes reflect other events we have going on, such as a planned visit to the mountains or a weekend of berry picking. Other times we focus the themes on a favorite subject or something the kids have had questions about.
Our first theme this summer was ducks, mainly because we had just added three runner ducks to the family. Each Monday during the summer we visit the library. The kids get the book they want plus a few that cover the week's theme. We then spent the week reading duck books, making duck crafts and other ducky things. We didn't do these things non-stop, just when boredom looked like it was about to strike.
Sometimes the kids get really into a particular theme, like caterpillars, which was a popular theme last year. When that happens, the theme can stretch over several weeks. We raised caterpillars into moths and butterflies, went on butterfly-spotting nature walks, and studied the monarch migration, all for fun. These summertime themes add structure and keep the kids engaged for their three month break, but since they are just for fun, they don't carry the stress and planning necessary for regular school year lessons.