If you kill almost everything, skip the charming wooden box and the thirsty terracotta pot. The most low-maintenance urban planter box is a self-watering, wicking planter with a generous water reservoir, an overflow hole, and a water-level indicator. Look for one made of UV-resistant recycled plastic or fiberglass. It will not rot, crack, or dry out as fast as wood or clay.
A 24- to 36-inch rectangular box with casters is ideal for a balcony, patio, or rooftop. Fill it with lightweight potting mix, not garden soil. Add a slow-release fertilizer and a thin layer of mulch. Then plant tough, forgiving choices like sedum, rosemary, thyme, lavender, ornamental grasses, or succulents. If the spot is shady, try snake plant, ZZ plant, or pothos.
The reservoir lets roots drink for one to two weeks, so your main job is checking the indicator and topping it up. Empty the overflow in rainy season, prune dead leaves, and refresh fertilizer once or twice a year. That is about as close to a no-fail urban planter as a black thumb can get: durable, self-watering, and planted with survivors.