I've leveled several park benches on bumpy grass and dirt, and concrete really isn't necessary for most situations. The key is to stop the legs from sinking, give them a firm drainable base, and make small height adjustments with shims or adjustable feet. Here's the method I use.
You'll need a shovel, rake, hand tamper or scrap 4x4, a 2-foot or 4-foot level, angular crushed stone or clean gravel, pressure-treated wood blocks or plastic paver pads, composite shims or thin PVC trim, adjustable bench feet if the legs can accept them, a rubber mallet, tape measure, gloves, and safety glasses.
Start by placing the bench in the exact spot. Put a level on the seat from left to right and front to back. Note which legs are low and which are high. On a slope, don't fight the whole hill. You only need to create level footing points under each leg.
Use a shovel to cut out sod and loose soil under each leg. Dig slightly deeper under the high-side legs so all four footings are close to the same elevation. Usually that means digging about 1 to 3 inches, unless the slope is steep. Remove soft black soil until you reach firmer ground.
Fill each dug spot with 2 to 4 inches of angular crushed stone or clean gravel. Tamp it down hard with a hand tamper or a scrap 4x4. This base drains water and prevents the bench from sinking into soft ground. Check your level often, and add more gravel under the low-side legs until the seat reads level both ways.
Instead of concrete, place each leg on a solid pad. Good options are plastic paver base pads, pressure-treated wood blocks cut to roughly 8x8 or 12x12 inches, composite decking scraps, or stiff rubber paver tiles. These spread the weight and keep the legs out of wet soil. If one corner is still low, add composite shims or thin PVC trim strips. They won't absorb water or rot like plain wood.
If the bench legs can accept them, install heavy-duty adjustable feet or threaded leg levelers. Drill a pilot hole in the bottom of a wood or metal leg, install the threaded fitting, and turn the foot until the bench is level. This is especially useful because the ground settles over time, and you can relevel in a minute without digging.
Once the bench is level, anchor it so it doesn't slide. Drive galvanized earth anchors or long landscape staples over the leg pads or through the bench frame if possible. If you don't want anchors, bury each pad about halfway and backfill around it with compacted gravel.
Finally, sit on both ends and the middle, then check the level again. The soil may compress a little under load. Add a thin shim if needed, tamp the surrounding gravel, and backfill. Recheck after heavy rain or winter, since freeze-thaw cycles can move the ground slightly.
This works for metal, wood, and recycled plastic benches. It usually takes one to two hours, and you don't have to mix or wait for concrete.