That's a fantastic and very practical question! Maintenance is often the hidden cost of outdoor furniture, so your concern is spot-on. The good news is, several bench types are champions when it comes to easy cleaning and low upkeep. The key factors are material and design.
For the absolute easiest clean-up, focus on non-porous, solid surfaces. Polyethylene (Poly) Lumber Benches are top-tier. Made from recycled plastics, they resist stains, moisture, and insects. A quick rinse with a hose or a wipe with a mild soap solution is usually all it takes. They never need painting or sealing.
Close runners-up are powder-coated metal benches (especially aluminum) and solid teak benches. Powder-coated metal has a smooth, sealed finish that sheds dirt. A damp cloth does the job. While teak is a wood, its natural oils make it highly weather-resistant. It weathers to a silvery-gray naturally, requiring no stain. For cleaning, just use a soft brush and mild cleaner.
Benches with simple, slatted designs are easier than those with intricate patterns, as dirt and debris have nowhere to hide. Avoid complex wrought iron scrollwork or rough, porous concrete if minimal cleaning is your goal.
For routine care, a simple regimen works best: sweep off dry debris weekly, give a gentle wash with a mix of mild dish soap and water a few times a season, and rinse thoroughly. This prevents buildup.
In short, for the least fuss, choose a bench made from poly lumber, powder-coated aluminum, or quality teak with a straightforward design. You'll spend more time enjoying your garden and less time maintaining it