We do a couple of camps - older ones go to a sleep away camp - very old school, swimming in lake, canoeing etc. That's the highlight of their summer. Love that there are no electronics.
Plus one other - where they pick the theme - girl ones include zumba, dance, swimming lesson, crafts, etc. or you can do all sports, or you can do a hockey one. That's a full day camp for a week. It's at our local rec centre and very affordable.
We camp some weekends in a tent. Old school. Kids love it. Or just set up in backyard if we can't get away.
We have pool so usually invite kids over at some point. But you could go to a pool. Meet up with friends or your kids will see some there.
Same with parks. I would take mine early morning (to avoid heat) and even if we didn't know the kids, they'd end up sliding together, etc.
Library, etc. kids always around.
Mostly unstructured. I am not big on structure with kids because in this day and age, with activities starting in Kindergarten (whereas when we were young we mostly started sports etc. later) OUR time as family is unstructured.
Some days we just take a walk and I'll give them a list of things to look for (the younger ones). Come home they glue it on paper. It's not structured but I guess they are doing something art-like and it keeps their printing up over summer.
Lots of biking (again kids run into other kids on bikes), and if husband can be flexible we'll head to beach on nice day.
I like camps. It breaks it up.
We also have done sports throughout summer - just recreational - not competitive. This year kids want to do drop in hockey - ice hockey and a bunch of them are doing road hockey at local park. Older kids scooter - so will drop off at skate park. Some summers we did soccer two evenings a week. Mostly they don't want to do sports in summer though unless it's just drop in. They like the break - busy through rest of year.
Enjoy your summer :)