Here's a thing we did a couple of summers ago:

We made a solar cooker out of a windshield screen, a bucket, some clips, a grill grate, and a stick. The windshield screen was already in my garage. It probably cost $8 or $9 at the auto parts store. I also already had a bucket and a grill grate from a tabletop grill.

I used the clips to hold the window shade in a cone shape, put the point of the cone in the bucket, and put the grill grate over the top of the bucket to hold the screen down and the pot up.

We tried cooking in a cast iron pot with its lid on (above), but I think that just took too long to heat up. Instead, I used an oven bag to let light in, but prevent wind from taking away our precious heat.

The temperature of the cheese hit about 160° F in the sun. You'll melt cheese easily, but there will be no browning. I'd say anything you could do in a slow cooker on low ought to come out fine on a sunny day.

We cooked a pizza on a pre-baked crust. I thought it was soggy and weird, but Danger declared it delicious.

Then we cooked sweet Italian sausage in a red sauce for a few hours. It came out perfect!