I know how frustrating this is—playground benches seem to grow legs, especially in quiet areas. You don't need to drill permanent bolts into concrete to make them a lot harder to steal. I'd use a layered approach: slow thieves down, make the bench awkward to move, and add a deterrent.
Here are the most practical no-bolt methods I've seen work.
1. Heavy-duty cable lock to a fixed object
If there's a tree, metal light pole, fence post, or playground frame nearby, get a thick vinyl-coated steel cable—at least 3/8-inch diameter—and loop it through the bench frame, then around the fixed object. Secure it with a weatherproof padlock or shrouded shackle padlock. It won't stop someone with an angle grinder, but most bench thieves are opportunistic and won't want the noise and time.
2. Weighted planter or concrete ballast
Attach the bench to a large heavy object that doesn't need to be bolted down. A big precast concrete planter, a steel drum filled with sand, or a concrete ballast block can add 200-400 pounds. Use stainless steel strapping or heavy chain to tie the bench legs to the weight. Hide the weight under the bench or plant flowers in it. The bench becomes too awkward for one or two people to lift quickly.
3. Screw-in earth anchors
For grassy or mulched areas, use large screw-in dog tie-out anchors or earth anchors. Twist them into the soil until only the eye is above ground, then attach the bench with a short hardened chain and lock. They're removable if the park changes its mind, but they hold very well and don't require concrete or permanent bolts.
4. GPS tracker hidden in the frame
Hide a small waterproof GPS tracker under the seat or inside a hollow frame tube. Add a small engraved plate: "This bench is GPS tracked." Thieves often avoid items that can be traced. Just check the tracker battery every few months.
5. Motion-activated lighting or cameras
Many benches are stolen at night. A battery-powered motion light near the playground, or even a realistic dummy camera with a blinking LED, can be a strong deterrent. If the area is dark and hidden, simple visibility makes a big difference.
6. Choose heavier replacement benches
If you're replacing stolen benches, avoid light aluminum or thin wood slat benches. Look for heavy recycled plastic, thick steel, or benches with built-in ballast bases. Some commercial park benches are designed with anti-theft weight and hidden anchor points.
7. Work with your local parks department
If this is a public playground, loop in the parks department or municipality. Ask if they can install removable ground anchors, add concrete footings, or register the benches. In many places, any modification needs their approval, but they may be just as tired of replacing stolen benches as you are.
The best result usually comes from combining two or three of these. For example, cable-lock the bench to a fixed post and add a weighted planter plus a GPS tracking label. A thief wants a quick lift, not a ten-minute noisy job with tracking risk. You'll never make it impossible without bolting it down, but you can make it enough of a hassle that it stays put.