Downtown isn't a shopping neighborhood the way SoHo is. There's no strip of storefronts to wander along here, because the Financial District was built for offices and the streets are narrow and shaded. What it has instead is two big indoor centers built into transit hubs, both within about a ten to fifteen minute walk of John Street, plus a handful of smaller pockets worth knowing.
The practical upside is weather. You can shop for an entire afternoon down here and barely go outside, because the World Trade Center complex is connected below ground. That matters in February, and it matters in an August thunderstorm.
Two things worth knowing before you go. First, clothing and footwear under $110 per item are exempt from sales tax in New York City, which is a genuine saving on a few t-shirts and a pair of sneakers, and it applies per item rather than per receipt. Second, the hours down here follow the office rhythm more than the tourist one, so the big centers keep normal mall hours but the small independent places often close early and skip Sunday.
Here's how we'd rank them, with walk times from John Street.