How can I upcycle old square duct pipes?

Ductwork is galvanized sheet metal. It will weather fairly well. It also bends easily, so you can create a variety of things from this. I would disassemble the rectangle first. They are generally two halves that pop together with little "buttons" on the edges. Use a flat screwdriver to release these buttons and it will come apart. Then you can flatten the metal and start fresh! Wear gloves! Galvy is extremely sharp like a knife.

Bonnie, if you click on "explore projects" then type "pipes in upcycle" in search box and hit enter you will find a good number of ideas. I chose one for you, hope it helps: https://www.hometalk.com/diy/outdoor/garden/diy-waterdrop-solar-lights-29722600

DIY Waterdrop Solar Lights

Please wear protective eyewear and gloves when handling metals.

With additional parts or resources, like standard aluminum flashing, You can make mostly anything out of these:

Fountains, Posts, Drainage, Roofing, Drain Pipes...etc...

Concrete molds, Paver Molds, etc..

Flower Pots, Raised Gardens, etc..

Galvanized metal is all the rage right now for the country chic/industrial look in decorating. You could use the materials to make signs that you stencil or hand letter. Also you might be able to make some very cool planters for inside or outside your home. Maybe reinforce with wood to make window boxes? You might be able to fashion them into some unique downspout extensions for your home too.