Pavement chalk is good. A bucket of water, an empty bucket, and a bunch of things to transfer the water between the buckets – colander, bits of pipe, guttering, funnel, plastic jug. My kids had some plastic stepping stones I picked up at a car boot sale and they made up lots of different games using them. Tray with soil in and build a bike jump track for a play mobile bike. If you have a suitable tree branch, weaving a rope through an old bike rim makes a nice basket seat/swing for just sitting outside and playing the things you might do indoors outside. If you have space, assign a corner of the garden for digging a hole.
My poor kids – they’ve had years of this stuff inflicted on them!