Errr, prob a bit obvious and not summery but Harry Potter?
Went to a Hogwarts party yesterday and my friend did Herbology (what you can eat from her garden - mint, strawberries, rosemary, etc but you may want to check out your garden first!); potions - her DH was Snape and the 'potions' were trifles the children put together themselves but you could also do experiments such as cleaning coins in salt and vinegar, making volcanoes/explodng potions (?sodium bicarb and vinegar), even make bath bombs (check out web recipes).
Make witches' and wizards' hats (paper cones and plenty of glitter/sequins and glue) or wands (wooden dowel and above).
Or completely different keep it very simple - sandpit, paddling pool, crawl through tunnels, dressing up races, potato races, egg and spoon, Twister, pavement chalks, hopsctoch, Little Tikes toys. Could everyone bring a slide/Cosy Coupe/similar? Do you have play equipment in your garden? Are they likely to get bored? Do you have a local toy library you could get equipment from?