Short answer: yes, you can add a small table attachment to a park bench for coffee or a laptop, but the best approach depends on the bench, the park, and how permanent you want the setup to be.
If you just want somewhere stable to set a coffee cup or a 13-inch laptop, the easiest option is a removable clamp-on tray. These are often sold as RV or wheelchair tray attachments, but they work on many wooden or metal park benches with flat armrests or slats. Look for a clamp that opens wide enough for the bench board and has rubber pads so it does not scratch or dent public furniture. Some are designed as small side tables that swing out of the way, which is useful if you are sharing the bench or need to stand up without moving the whole thing.
A portable folding laptop table or a small camping side table can also do the job without attaching anything to the bench. You simply set it in front of or beside you. This is often the safer choice in public parks because it leaves no marks and does not look like you are modifying city property. For coffee, a clamp-on cup holder is even simpler, and many are made for boats, strollers, and wheelchairs; they attach to armrests or seat edges in seconds.
Before you clamp or strap anything to a park bench, check the local park rules. Many cities have regulations that prohibit attaching objects to public benches, tables, or trees, especially if the attachment could damage paint or wood or interfere with other visitors. In practice, park staff usually do not mind a temporary, non-marring accessory that you remove when you leave, but if a sign says no attachments or no structures, respect it. The key words are “temporary” and “leave no trace.” If you would not want someone drilling into a bench near you, avoid anything that requires screws, nails, or permanent adhesive.
If you want to go the DIY route, you can build a lightweight bench table from a wooden or plastic tray and two C-clamps. Attach the clamps to the underside with short bolts, add rubber or felt pads to the clamping surfaces, and test it at home on a similar board. Make sure the tray is not so wide that it blocks the walking path or hits someone sitting next to you. Twelve to sixteen inches of table surface is usually enough for a cup and a small laptop or notebook. For stability, place the clamp over a sturdy board, not just the edge of a slatted seat, and keep heavier items centered over the bench rather than hanging off the outer edge.
One more practical point: park benches are rarely level, especially on grass or dirt. Your table will likely wobble unless you use a clamp with adjustable tension. For laptop work, bring a lap desk or use a backpack as a support under one side. Also remember that direct sunlight can overheat a laptop and make a screen hard to read, so pick a shaded bench if possible.
So yes, you can absolutely add a small table attachment to a park bench for coffee or a laptop. The most realistic setup is a removable clamp-on tray or a portable folding table that gives you a stable surface without damaging the bench or breaking park rules. Keep it light, keep it temporary, and clean up any coffee drips before you leave.