We built a vegetable wash station/play kitchen out of found wood (upcycled plywood or scraps from other people's projects). It has a 5 gallon bucket hanging above it for rainwater or hose water and a faucet from our local recycled hardware store, so it's functioning gravity fed wash station. We also put in a play stove and oven just for fun. Although if you're really crafty, you could make those functioning as well...in a kid friendly way.
We also have a huge trellis thing for our squash to grow up. It looks like only of those octagonal climbing structures at the playgrounds, but it's also made from upcycled 4x4 lumber (definitely not as pretty or geometric as the playground version), but the squash climb up the inside, and the kids climb up the outside.
Or you could grow the squash on the outside and the kids can have a dark(ish) fort on the inside. We have an old tire turned into a tire swing, a few other tires and a ladder made into an obstacle course, a pallet swing/bed under the grape arbor and a huge slip n slide made from all found materials (old vinyl tarp material 4'x15', 2 - 4"x6"x15' sticks on both sides of the vinyl to keep the water in, a slide made from 2 pallets - one side ladder, the other side covered in leftover laminate flooring for the slide (super slippery)....best thing ever for the kids during the summer!)
We also planted a few plants we didn't really care about throughout the yard so they could have something to grow all by themselves. I have a 2 year old and I have a small daycare for toddlers through age 7, and these things plus all the animals to help feed and water, they're pretty happy.
I wanted to build a full play structure/castle thing as well, but I'm a single mom with toddlers running around all the time, so I did break down and buy someone's old plastic play structure to add to the rest.
Not too excited about having it, but it wasn't bought new and the kids enjoy it, so we'll live.