I purchased a Nikon TC-E15ED 1.5X Telephoto Converter Lens for my 5700 about a year ago, and I absolutely love it. It brings the built in lens of the 5700 out to the 35mm equivalent of 420mm. The optics are perfectly matched resulting in tack sharp images from corner to corner. It's a fairly small lens, and the additional weight (a mere 9.7 oz) actually balances the 5700 perfectly. I have handheld 420mm images at remarkably slow shutter speeds (1/90th of a second).
This lens is theaded for standard 58mm filters. It works particularly well for my setup because I also use 58mm filters on my 5700 utilizing the Nextphoto Adapter.
The only slight downside to this lens is that you can only use it at maximum telephoto. None-the-less, I think it's a great accessory for around $150 in the United States.
My Nikon 5700, UR-E8 Adapter Ring, and TC-E15ED 1.5x Telephoto Converter.
My Nikon 5700 with the UR-E8 and TC-E15ED mounted.
This setup gives me the 35mm equivalent of a 420mm lens!
Useful Tip: The TC-E15ED comes with a very nice soft lens pouch for storage. Additionally, the lens pouch will hold the TC-E15ED with the UR-E8 adapter ring still attached. To top it all off, the pouch will then fit right into the bottom of the Tamrac 5696 Case, with plenty of room for the 5700 to fit above it. (The filter case, which I had previously stored in the bottom of my Tamrac case, clips nicely onto the shoulder strap.) The result is the same ulta-compact camera package that I have grown to love while hiking and exploring in the great outdoors.
If you love your 5700, you owe it to yourself to purchase the TC-E15ED. I strongly recommend this product!