A comprehensive review of Nikon's entry level DSLR from Imaging Resource:
The Nikon D3400 combines great image quality from its 24-megapixel APS-C sensor with a user-friendly design and good, reliable performance across the board. Some of its capabilities may be lacking, particularly with regard to continuous autofocus, video and its new SnapBridge support, but compared to its peers, the D3400 still proves to be an excellent entry-level DSLR at a great price.
I have to say; I find this a fascinating little camera. With all the attention on Mirrorless cameras, these days people often dismiss these entry level DLSRs as nothing but the "same old same old". Spec wise this is an impressive little camera, particularly in the price range. When you consider it has a 24mp sensor, and it is a relatively small body, if you're looking for a small companion camera, it might be worth considering something like this alongside mirrorless options. It has compatibility with the full range of Nikon lenses, has a sensor comparable to the Sony A6500, has a relatively small body, and costs less than $500.
Sure it may not have all the features of some of the Mirrorless options, but if you just want something that takes pictures as a second camera, and especially if you're already a Nikon shooter, you should take a look at this, and the larger 5400.
Just a thought. (And I love mirrolrss cameras by the way, so it's not an anti mirrorless thing)