Good boy: Make Vitamin D (and probably a host of other synergistic co-factors)
Mood - it just does a mind good
Tan - protects against burns and looks nice
Bad boy: Can cause burns, sometimes very severe ones
Possible skin carcinogen
In my mind at least, it's not too hard to come up with a simple and effective (but maybe not always easy) plan to find a good balance of sun time.
Get frequent sun exposure on your bare skin
Don't burn your skin
Wear clothing to protect your skin for prolonged exposures
Only use sunscreen when absolutely necessary
A little more detail here perhaps. For sun exposure, ideally you need to get it on a large chunk of your body. Your bare skin showing while wearing a t-shirt and shorts is better than nothing, but try to shoot for shirtless if at all possible (see, not so easy). Also, you want to be under the sun in the late morning or early afternoon. This has to do with the 'angle' of the rays as they come through the atmosphere and penetrate your skin. A good rule of thumb here is to be in the sun when your shadow is roughly your height or smaller. Not coincidentally, this 'angle' thing is also why we don't make vitamin D from sun exposure in increasingly Northern/Southern latitudes in the middle of winter. One more detail: progressively build up your sun time in the spring so as not to burn yourself.
As with anything, you can make this a lot more complicated if you so desire (not recommended). For instance, you can debate about what optimum serum vitamin D levels are and if you should or should not take supplements to reach some arbitrary threshold. And you could try to figure out how long your sun exposure should be depending on your skin color, time of year, and even things like altitude in my case. Or if you do need to use sunscreen, what type (chemicals or no chemicals, UVA and UVB protection or not) and SPF strength.
Instead of going crazy, remember that the sun should be fun. Respect it, but don't fear it. Use it, but don't abuse it. Enough said right? As I found out in Portland, not so much. My niece and nephew essentially don't get non-sunscreened sun exposure ... by their parents choice. Even the week I was there when it was mostly cloudy and rainy outside, sunscreen would be applied copiously.
To be sure, I understand what's going through the mind of my bro and his wife. It's not hard to figure out. Avoid sunburns for the kids at all costs. I get that. You get that. My friend's pet turtle gets that. But what I don't get is how they cannot see the benefit, nay, the absolute importance of some sun exposure, or the downside of sunscreen (which I admit is not so common knowledge in conventional wisdom circles). Unfortunately, the kids being kids, they don't get vitamin D in their diets (I'm purposely not counting the synthetic D in the 2%! milk they drink - oy vey). In other words, they don't eat eggs or fish and such. Not that this would or could make up for a lack of sun exposure. I'm actually not going to get into their diets here, that's a subject I'll just leave alone. It pains me to see what they eat, but it's not under my control ... especially with my sister-in-law being a registered dietician. And you know what? I didn't say a word to them about sun exposure either. If I thought it would have left even a seed of doubt in their minds I surely would have, but they see old uncle Aaron as on the fringes. The fringes of insanity probably.
Ah well, what can one do? Enjoy the heck out of family time while I'm there is what. Have catches and conversations and read the kids a book before bedtime. Sing songs, do sprints together and go out for ice cream. That's the good stuff right there. If I ever have kids of my own, I'll undoubtedly feed and sun them differently, but that's me. When my niece and nephew get older and begin to make choices for themselves, hopefully, the conventional wisdom will have shifted some by then. It has started already, I can feel that much at least.