If they really don't want to do story summaries, I'd let that one go. Why? Because you are asking them to entertain themselves, and this is one where you are going to have to 'put your foot down'. It also detracts from the fun of reading, and they might spend longer at it if they don't feel there's an onerous task ahead. Let that live in their Sept-June 'school' world. Could they summarize at the lunch table, or at dinner, so it's less formal?
I love the coin counting idea. I did a coin activity with children a few years ago, which included noticing mint marks, years of coins (which is the oldest? the newest?), noticing the many different pictures on coins (why does each state quarter have the picture it does?... there's a good 'research' one) and even had the children draw their own 'commemorative coins' on paper. They loved it.
Heraldry (coats of arms, symbols) is also another big fun one with a lot of opportunities for creative play and extention. With a good book or some downloaded info, each child can create their own 'coat of arms', crests... with one group, they were given paper and string to make rows of these symbols to festoon on cardboard box 'castles'. Some children at this age are very excited about knights, princesses and dragons.
I love the idea of taking lots of pictures and journaling. Maybe get them clipboards for local educational outings and let them document/draw/create a game based on where they are and/or what they are looking at. Sidewalk chalk works for this too. Creating games challenges multiple areas of thinking.
You could also do some fun stuff with poetry. Poetry line-find: give them each one ambigous line of poetry from their books,and have them search for the line and it's meaning. Judith Viorst has some wonderful kids poetry, and I know the library is full with good poets that are fun for kids to read.
I also wonder if some of the presentations could be more fun if they were done through dramatic play. This sort of production could be a 'show' for visiting family or friends. Kids love to perform.
Would they want to keep up a newspaper during the summer, where they can show off their new knowledge of their town, any kid-appropriate current events?
My son is four and I'm a preschool teacher, so we do have some hidden educational agendas going on this summer. Visits to the zoo, fountain/water observations, (he can reproduce certain fountains with blocks, already) trips to watch the big machines, wherever they might be. I will be encouraging him to find a "letter/number of the day" to boost recognition, so we'll pack a stack of index cards with those on them in the backpack.
We have a sandbox which will be covered, and endless water play. Lots of science opportunities!
He's also crazy about dinosaurs and I've got some fun ideas: going to the playground and measuring out how long/tall a couple species of dinosaurs are on a given trip and drawing them out with chalk; drawing out a few dinosaurs with numbers or letters on them for a "run to the letter/number game". Usborne makes some great science books with loads of kid-friendly experiments (they involve safe materials) and one poster here has a site for the same:
http://topscience.org/
I'd encourage you to check it out. I plan to use some of the experiments myself. Kids really learn more from the hands-on work than from secondhand observation, from my experience.
Other than that, I've got an unschooling bent in that I want my son to play and learn at what he is interested in. That could mean "How many dinosaurs are going to fit into your Tonka truck?" and making predictions, or
"what will happen if we try to empty the bucket of sand into the bucket of water? Will it fit?" Ramps, cars, blocks, singing his made-up songs... I can't wait.:)