If you don't know how, or don't have the time to adjust photos for best looks, you need help. Here's a tool that will do it for you.
LUCiD is an app that analyzes photos and tweaks a lot of parameters to make them look as good as possible. As good as possible in this case means natural. It doesn't add glow and other effects, just makes them clearer and with better colors. The changes often are very subtle.
In addition to a one button "fix all" it has a button for fixing dark photos, one for tint, one for noise and one to beautify portraits. All fixes are manually adjustable by choosing an adjustment and swiping left or right over the screen.
You can use LUCiD as a standalone app or as an action extention. I like the extension better because it usually is faster when I'm in Photos.
For an example of LUCiD in action, look at the enclosed images. The darker one is before and the lighter one is after LUCiDs adjustments.
The LUCiD app cost 3 USD in the App Store and works on iPhone as well.