I'd start by treating slip resistance as a material and drainage problem, not just a style choice. Honestly, the easiest way to avoid a slippery disaster is to choose for wet conditions first and style second. Start by looking at the material and finish. Smooth metal, polished stone, glazed tile, and high-gloss composite are the usual culprits when rain hits. Instead, look for textured surfaces: brushed or powder-coated aluminum, perforated metal, slatted wood, woven resin, or composite with a deep woodgrain. If you're considering wood, teak, acacia, or iroko with an oiled finish usually offers better grip than a thick glossy varnish. For concrete or stone, choose a flamed, bush-hammered, or broom finish rather than a polished one.
Next, think about drainage. Water that sits on a seat is what makes it dangerous. Slats should be spaced wide enough to let water pass, and perforated or mesh seats should have holes that don't clog with leaves. A slight slope in the seat or tabletop helps, but not so much that cushions slide off. If the furniture is fixed, check that water can escape at the base, too. Hidden pooling under cushions can create a slick film.
Cushions are tricky. Vinyl and some coated fabrics can feel slick when wet. Quick-dry mesh, textilene, or open-weave outdoor fabrics are better because they drain and don't hold a water layer. If you want cushions, look for ones with ties, grip dots, or a slightly rough texture on the bottom so they stay put. Better yet, choose a sling or slatted seat for all-weather areas and add cushions only when you'll use them.
When you shop, test the surface if you can. Bring a spray bottle or wait for rain, then run your hand across it. Tilt the piece slightly to see how water behaves. A good wet surface should feel grippy, not sticky, and water should sheet off or drain away rather than pool. Also check the feet: rubber or textured pads can help, but they shouldn't trap water against the frame.
Finally, maintenance matters. Algae, moss, pollen, and grime can make even a good surface slippery. Clean regularly with mild soap, and avoid waxes or sealers that leave a glossy film unless they're specifically rated as anti-slip. In public or high-traffic urban spaces, look for furniture that meets local slip-resistance standards and has replaceable parts, because wear can change the surface over time. In short: choose texture over gloss, drainage over flat surfaces, and quick-dry fabrics over slick cushions. That's how you get urban outdoor furniture that looks good and stays safe when the weather turns wet.