Metal fire pits can rust in one season if they are constantly exposed to the elements. While they may seem unsightly or unsafe for a fire, they haven't outlived their usefulness. With a little elbow grease and some creativity, you can repurpose these rusted fire pits for other uses in the garden.
Starting with rough sandpaper and working your way down to fine-grade sandpaper, sand off as much of the rust as possible. Be sure to check the entire fire pit for thin spots that might weaken the structure of the bowl. If you find any thin spots, don't risk lighting another fire in it as the weight of the wood could cause the fire pit to collapse.
If the fire pit is only lightly rusted and most of it is intact, spray-paint the bowl with paint that resists rust and can withstand high temperatures.
Turn the fire pit as is into a planter. The depth of the fire pit may limit what kinds of flowers can be planted in it, but gardeners at a local nursery should be able to give you some idea of what kind of plants will thrive in the soil depth you have available. In addition to annual and perennial flowers, a fire pit can also make an excellent base for a rock garden that can include sand and cacti. If the pit has decorative shapes cut through the metal on the side, line the pit with a mesh screen to contain the dirt while still allowing water to drain through. If there are no such cutouts, drill several holes in the bottom of the pit to allow water to drain; standing water will rot the roots of many plants.
Turn the fire pit into a decorative garbage can for the porch or patio. Rather than using an unsightly plastic container, line the fire pit with a garbage bag and place it in an out-of-the-way location for a decorative alternative. Most fire pits have a lid to prevent sparks from leaping out of the fire, which will serve to keep insects and other creatures out of the garbage.
A small fire pit is generally large enough to contain the garbage generated by dinners on the back porch or other outdoor activities. The small size will also encourage family members to empty it frequently, cutting down on undesirable odors coming from a garbage bag baking in the summer sun.
The bowl shape of many fire pits makes them ideal as decorative places to store a garden hose. Place a small brick in the center of the fire pit to help keep the hose lying properly, and simply coil the hose around the brick. Drill a few holes in the bottom to let water drain through, and keep the cover on to prevent bees or other insects from choosing the repurposed fire pit as a place in which to build a nest.