Short answer: yes, in most cases you can replace just the wooden seat on a park bench with a metal one. It depends on how the bench is built, but many park benches are designed around standard slat sizes and simple bolt-on seats, so swapping materials is very doable.
Start by checking the frame. If the bench has a metal or wood frame with wooden slats bolted across it, you are in a good position. The replacement metal seat can be made from aluminum slats, powder-coated steel planks, expanded metal, or a pre-fabricated metal seat panel. The main thing is matching the existing bolt hole layout, or being ready to drill new holes.
Measure before you buy anything. You need the overall length of the seat, the width and thickness of the current wood, the number of slats, the gap between slats, and the center-to-center distance between mounting holes. Write these down and take photos. If the bench manufacturer sells a metal replacement seat kit, it may be a direct fit. If you buy generic metal slats, expect a little trimming or drilling.
Hardware matters more than people think. Do not reuse old rusty wood screws. Use stainless steel bolts, washers, and locking nuts. If the bench frame is steel or aluminum, stainless hardware is usually a safe choice. Adding a nylon or rubber washer between the metal seat and the frame helps reduce noise, scratching, and movement. If the frame is powder-coated, avoid over-tightening because that can crack the coating and encourage rust.
Keep comfort in mind. Metal gets hotter in direct sun and colder in winter than wood. Textured, perforated, or powder-coated metal is better for outdoor seating than smooth bare metal. Slats or perforated panels also drain water well, while solid sheet metal can hold water and become slippery.
The actual swap is fairly simple. Remove the old wooden slats carefully, clean the frame, test-fit the new metal seat, mark any new holes, drill slowly with a metal bit, deburr the edges, and bolt everything down. Do not fully tighten the bolts until all of them are started so you can make small adjustments.
So yes, you can absolutely replace just the seat with a metal one. Just make sure the frame is solid, take accurate measurements, use the right hardware, and choose a metal seat that works for your climate and comfort needs.