A cheap, remarkably entertaining activity for kids is a BYOB event. By that I mean Bring. Your. Own. Box.
We know there are significant downsides to too many toys and too many structured activities for kids. That's why a BYOB event is refreshingly different. It's an enticement to open-ended, creative free play.
My family has hosted a number of box parties. We tend to host them when something else is planned---a reunion or cookout or neighborhood get-together. We just add boxes into the mix. Here's how we do it.
Invite guests to show up with cardboard boxes of all sizes; the more the merrier, the bigger the better. We supply tape (it helps to have masking tape, duct tape, and packing tape), markers, washable paint, and plenty of room on our property. We try to get a few large appliance boxes in advance too. Repurposing yo!
Adults at the party are charged with the task of wielding mat knifes to cut where the kids direct. Typically the cutting phase doesn't last too long, which means the adults are soon free to sit around in lawn chairs noshing and conversing while the kids go wild with tape and paint. That's actually the prescription for a great party---happy, engaged kids and relaxed grown-ups.
Over the years we've seen kids collaborate on all sorts of different cardboard box creations. Sometimes more than a dozen huge boxes are transformed into tape-connected cardboard rooms featuring turrets and a rope-opening drawbridge. Sometimes kids create a connected series of tunnels leading to a fort under a tree. One time the kids made a child-sized maze they invited to adults to enter, giggling as we stooped and crawled and squeezed our way through. Another time they didn't connect the boxes at all, but cut armholes into boxes they wore as machine-voiced robots that comically misunderstood language and did a lot of bumping into each other.
The biggest thrill for kids seems to be in the planning, arguing for one vision or another before working together to make the project a reality. Of course, playing in it afterwards is fun too. The benefit of hosting it here? Plenty of days to play in the cardboard playland after our BYOB event is over.