What are their ages? Most of these are free or can be purchased at Dollar Tree type stores.
Picnics (inside and out), use those little plates with animal faces, breakfast with linens, etc eaten outside, scrapbook of summer activities - drawings or photos or both, play doh recipe projects using various shaped items, snow fight with socks, learning about money including savings and tithing - open up saving accounts and put dates on tithing envelopes rubber banded together), large chalk for driveway/sidewalk drawings, fashion show (boys usually want to judge or wear Halloween costumes or both - the announcer describes the outfits - this can be all their imagination while model walks up and down the special "runway" which can be silk flowers strategically placed, fashion show of pets, ten commandments and other Bible verses (write them on slips of paper and let them draw them out of a jar and tell the others about it), on freezer paper make a human timeline with corresponding pictures (act out as much as possible -make "drawings" and hang on the inside their caves (popup tents) for the Stone Age and for the last 200 years, they pick the things they're interested in pursuing to add to the chart on the freezer paper, movie night, practice fire drills, behavior modification such as when they insult the others they have to give them several sincere compliments, build a fort or tunnel or maze out of cardboard boxes, crafts such as using old magazines to make posters to learn the seasons or just their favorite things, papermache projects, "cooking" opportunities such as making fruit popsicles, jello (or make various flavors of jello ahead of time and let them "create" their own dish), make ice cream - throwing container to each other for about 5 min., science projects, put titles such as Scientist or Investigator on small pieces of paper and pin on them and arm with magnets or magnifying glass or rulers to see things in a different perspective, computer time - daily a few words of another language they select or identify various musical instruments using audio files, on grocery store trips - they pick an item that they've never eaten before, trips around town pointing out businesses and describing what type of jobs are done there and later play "pretend" to act out those jobs, duplicate "emotions" chart in the mirror and identify, and have a costume box which can be out of style clothes or torn linens. Introduce a new food by raising their interest - once the food is on the table, drape a plastic tablecloth over it then let them know it's time to eat and let them reveal the dish. Also, select several personal characteristics that I want to encourage or help them see in themselves and increase as summer passes. Lastly with kids in a circle with their backs to each other, toss balloons to each other.