Short answer: most standard park benches do not come with legs you can simply twist or extend to match uneven ground. The legs are usually fixed, whether they are welded steel, cast iron, or pre-cut wood. That said, you are not stuck with a wobbly bench.
If you are placing a bench on grass, dirt, or slightly sloping pavement, the easiest fix is to add self-leveling feet. Many outdoor furniture suppliers sell threaded leveling feet that replace the existing foot caps or screw into the bottom of the legs. This is a simple and effective park bench stability upgrade. These adjustable feet let you raise one side by a quarter inch or more until the bench sits solid. Stainless steel or zinc-coated versions hold up best outdoors.
For smaller gaps, rubber shims or composite leveling wedges work fine and are almost invisible. For uneven ground leveling on a permanent installation, consider setting the legs in concrete footings or using recessed leg mounts so you can level the base before bolting the bench down. Some commercial park bench models do offer adjustable park bench legs or leg extensions as an accessory, so it is worth checking the manufacturer’s spec sheet before you buy.
If you already own a bench and it rocks, try tightening all fasteners first, then check the ground. Place a level across the seat and lift the low leg until the bubble centers. Measure that gap, and choose a leveling foot or shim that matches. With the right hardware, a proper outdoor bench installation can give you a rock-solid seat on almost any surface.
Need more specific advice? Tell me the bench material and whether it will be anchored or freestanding, and I can point you toward the best leveling option.