I will give you my top three based upon the amount of popcorning
that my pigs have done in response to my random cage additions. Also, I'm not too familiar with what a Midwest cage is, but I searched online and it looks like there are a couple different types. However, all of my toys can be modified to work with most any cage.
1.
Fleece forest. Buy some fleece, cut strips, tie them to a cube that is above the pigs' heads (I tied mine to the bottom of the second level, so when they're on the first level they run around in it). If you click on my name and go to my album, there is a picture of Grizzly in the fleece forest for reference. Since your cage (from what I saw on google) doesn't have a top, you could always punch holes in cardboard, feed fleece through the holes, binder clip the top of the fleece to prevent it from getting pulled through, and just set the cardboard on top of the cage edges (or binder clip it to the cage edges).
2.
Corner towel. I binder clipped a small hand towel over a corner of the cage. It has basically become a nap spot/necessary checkpoint in their zoomies. They love to run around under the towel, as it makes a sort of tent/tunnel in their corner. Again, very basic addition that you could do in 2 minutes. If you need a picture let me know and I can take one later. If the corner isn't enough of an angle to keep the towel from just laying flat against the edge, you can move a pigloo closer to the edge and drape the towel on top of the pigloo, leaving enough room between the pigloo and edge of cage for running under the towel tent/tunnel.
3.
Tunnel with hay. My pigs love their hay and their tunnels. Try stuffing a tunnel (or toilet paper roll that is cut) with hay and leave it in their cage. You can also hide a treat in a stack of hay to stimulate them.
And my wildcard pick:
Ball of newspaper! So easy, but Yeti loves to grab this and run around with it all over the place.
Good luck! Try getting creative with anything that won't harm them through contact or consumption and you should have some very happy pigs.