I discovered through a thread on the DPReview Forums (about wishful features for the Coolpix 5700) that you can get a thread adapter for Coolpix 5xxx series cameras to allow filter attachments.
It’s just some guy out of Canada that machined (?) some thread adapters on his own. And, they're available for the Coolpix 5000, 5400 and 5700 in several sizes (46mm, 49mm, 52mm, and others).
I wanted to get one, but at first I wasn’t sure which adapter size would be best. Apparently, 52mm adn 58mm are two of the most common sizes for filters, so that narrowed it down. And, I was concerned that a 58mm filter might get in the way of the built-in popup flash, so I went with the 52mm.
I also picked up an Excellent-rated used Tiffen circular polarizer from KEH to go with it. If you’re not familiar with polarizers, they’re most commonly used to enhance blue skies and, as a rule of thumb, autofocus cameras need circular polarizers (as opposed to a linear polarizers).